My grandfather (who, a month younger than Elliot Carter, will celebrate his hundredth next month) grew up on a ranch in Union, California (which no longer exists, having long since been incorporated into metropolitan Paso Robles). At harvest time, they had three men and twenty-one horses attached to the harvester. One man sat on the lead horse, the second tended header, and the third, my grandfather, watched the rest of the horses, got the harvest into sacks, and sewed the sacks shut. He was the San Luis Obispo County Fair champion sack-sewer for several years running, and in-between sacks, he'd kill birds with stones and a sling (not a slingshot, just a bandana and some loose rocks). Nowadays, the same amount of work would be done, in a fraction of the time, by a single worker sitting air-conditioned in the cab of a big piece of motorized equipment, probably listening to canned music all the while. I assume that birds are scared away by other means.
With the classical/romantic concert and opera orchestra, there has been no equivalent scaling-down in labor requirements, which have generally been limited to humans and have generally excluded the equestrian. In the popular theatre orchestra, there has been significant scaling-down, to the point now in which a theatre orchestra is usually much reduced, a small band with synthesizers imitating larger sections of instruments and, indeed, sometimes the pit is empty, live instrumentalists replaced by a recording. Others more expert can speak to the case of the theatre orchestra, but the relative stability of the concert and opera orchestra deserves some attention. In part, the stability at late 19th century levels is due to some historical inertia and many large cities are effectively moving into the practice of establishing smaller bands or sub-sections of the larger orchestras, with appropriate instruments and bows, for baroque and classical repertoire, and a few fortunate communities also have chamber orchestras which focus exclusively on 2oth and 21st century repertoire. But there remains music composed for larger forces which requires those forces for significant musical reasons. It might be surprising for some to learn that volume is not the critical issue, as masses of instruments, violins for example, playing the same part do not simply multiply decibels with their numbers. In fact, the addition of instruments to a section creates the "chorus" effect, in which the individual instruments create many small variations in pitch, timbre, dynamics that are nevertheless coherent in structure thus synthesizing a lively, detailed, sound with a diffuse physical presence but not actually an increase in volume proportional to the numbers of instruments. In this manner, a large string section can play pianissimo, on the one hand, and on the other, at the highest volume, still permit an independent solo line to be heard against the mass. Magical stuff*, and essential to some musical styles, but it comes with a real labor cost.
The question is how does one best afford to continue to support this very special musical quality in times in which institutions are perpetually struggling with labor costs. (The recent efforts, by German communities, to de-couple orchestral musicians from civil service pay schedules in one step in this direction). In part, smarter orchestral managers will be able to use the tripartite classical/romatic/modern orchestral structure described above to more efficiently assign their resources (which also has the benefit of automatically covering a wider variety of music), but such efficiency is no substitute for making the case that the music requiring larger forces is a cultural good, a unique aesthetic and social product, without which we are impoverished and far less interesting as a species.
* I write this as a (mostly) disinterested party. I have not, under my own name, written for larger orchestral forces and have no plans to do so, as the economics of a commission for an experimental work for large orchestra simply do not work, as the possibilities for repeat performances are usually limited. This is a situation with which I can live. My visits to the larger orchestral world, on-hire orchestrating or ghosting for film or concerts, have provided adequate, if vicarious, pleasures of the sort.