Looking at the action (moving parts) of a modern grand piano, one can easily draw two mistaken conclusions: that such a mechanism is very complex; and that pianos must have evolved fairly recently to their present state of perfection, so that the instruments of the great composers must have been very primitive and inferior by comparison. Indeed, the literature available on the history of the piano leads one to see the instrument's development in terms of progress and innovation, with the modern piano as its high point. One often hears the sentiment that if "poor old Beethoven had had a fine modern piano to play on, he would have been delighted. What a pity he had to make do with a piano that couldn't express the majesty of his music!"
Let us look more carefully at history, to see if these impressions are accurate. First, the matter of technology: during the Middle Ages, clocks were built that chimed on the hour and half hour. By the late 1500s, clocks made for the amusement of noblemen featured such mechanisms as little figures that marched around beating drums on the hour. By the late 1600s, pendulum clocks could keep accurate time to within seconds per day. So in Florence, Italy around 1709, when Bartolomeo Cristofori built his first piano that is known of, the kinds of engineering involved in building a working piano mechanism would not have been regarded as high technology. Cristofori continued to refine and develop his designs so that by 1726 he had worked out most of the problems, and created a fairly sophisticated, working instrument. The piano-forte (literally, "soft-loud", since it responded to the weight of the player's touch) was regarded as an interesting gadget at the time; people were quite content, however, to continue using harpsichord and organ, which met the needs of Baroque music. As musical tastes changed, in the last half of the 1700s, the piano came into favor as the keyboard instrument best suited to the music of the day.
How does one explain the continuous changes in the piano throughout the nineteenth century? The main answer lies in the music. A musical instrument exists to make the kind of music people want to hear, and musical styles change constantly. As pianos changed to fulfill the requirements of these changing tastes, certain musical characteristics were no longer given the same priority, and were given up or modified in favor of new ideals. Until well into the nineteenth century, piano performance was by the composers, who played only their own or their contemporaries' music, so there was no need to continue meeting the musical requirements of earlier styles.
Pianos built before the twentieth century frequently displayed intentionally wide ranges of tone color: an instrument might sound velvety when played softly, reaching a clarion brilliance at the loud end of its dynamic range; high and low registers on one piano might be like voices or instruments in small ensembles, sounding good together, but maintaining individual character. By the twentieth century, the trend in piano design was increasingly toward an ideal of perfect evenness. High and low pitched notes, soft and loud tones were made to blend, in a smooth continuum, rather than changing in character. This was consistent with Industrial Revolution goals of standardization and uniformity for all sorts of manufactured goods.
As much as changes in musical styles, the development of the piano reflected ideals of the Industrial Revolution, which glorified all sorts of new patent devices. Every piano builder vied with his competitors to invent new variants on piano action, special-effect pedals, frame design, and so on. While there was keen interest in what one's competitors were selling, there was not the urge to build a highly standardized product. Each shop had its faithful clientèle, just as different brands of product have their own adherents today. Therefore, two pianos made in the same city the same year might sound significantly different from one another, and feel different to the player's hands. Moreover, there were different national styles of piano building, each working well, but built on different design principles.
In reading a history of the piano, one must consider the source of information. If the research was based on the archives of an established piano-building firm, one may expect the material to favor that company as an important innovator, whose instruments were far superior to those of the competition. One important, and often unquestioningly quoted, early historian of the piano was an employee of a venerable piano company, who naturally saw change as progress, with his own firm in the lead.
Another consideration: by the late nineteenth century there was a new availability of printed music, and a new sense of there being a body of "classical repertory" for the piano. In our day, the piano repertory extends from Baroque music originally composed for harpsichord, to whatever anyone wants to compose at this moment. Since much of the standard repertory was composed for half a dozen or more distinctive forerunners of the modern piano, none of which was designed with the same ideals, the question arises, "What can a modern pianist gain from acquaintance with pianos of the past?" Although the modern piano cannot yield the same musical effects that were available on its predecessors, there are ways in which the earlier types of piano can inform the modern pianist's understanding of the music. Matters of balance between right- and left-hand melodies; pedal and metronome markings that make no sense in the context of the modern instrument; thick harmonization in the low bass; these and other musical details, when related to the appropriate piano, help the pianist to understand what the music was like in its original form, in turn pointing to more effective interpretations on the piano of today.
As to whether Beethoven would have liked the modern piano, one might as profitably wonder whether he would have liked the electronic synthesizer. While he may have yearned (especially as his deafness worsened), for an instrument capable of producing a larger, fuller sound, it seems improbable that he was imagining a type of piano tone that would not be developed until two generations after his death, any more than a composer today can predict instrumental characteristics of a century from now.
A final comment: if you have heard present-day copies of early pianos, or even restored original instruments, and have been disappointed with their tone, please bear in mind that both the modern copyist and the restorer have come to the early piano without the benefit of having grown up in the culture which produced the originals, or of having served a long apprenticeship with a master builder of the period; their sense of piano characteristics has been shaped by the modern piano. Several instruments in the Frederick Piano Collection, "restored" for former owners, sounded (as one pianist said, on hearing a new acquisition, just unpacked,) "perfectly NASTY!" Edmund Frederick's instrument restorations, based on a combination of solid historical research and a phenomenally discerning ear for timbre, are generally acknowledged to sound more beautiful than instruments heard elsewhere. Pianists and other visitors familiar with pianos in other collections have said repeatedly that they have been impressed at the musical beauty of the Frederick Collection. Audiences at the Fredericks' concert series return, season after season, bringing friends.