My understanding is that when writing for an orchestra, a composer needs to adjust to the orchestra's skill level and likely err a bit on the easy side for good measure.

Most composers, most of the time. Beethoven, on the other hand, refused to rewrite portions of the Ninth that were maybe too high for some to sing, and you can probably find difficult instrument parts if you chat to, say, Bassoon players.

Heh to quote Beethoven (from: https://brooklynrail.org/2007/07/music/third-party-beethoven...):

> As practice gradually moved away from this insular model, and standing, virtuoso quartets replaced ad hoc groups as the chief vehicles of chamber music performance, Beethoven’s compositions became increasingly experimental (and daunting). Legend has it that when Ignaz Schuppanzigh—a well-known Viennese violinist and Beethoven supporter—complained about a particularly difficult passage in one of the Op. 59 quartets, the master retorted, “Do you suppose I am thinking about your wretched fiddle when the spirit moves me?”

If you write a piece that "plays well", a player can play it well — and give a more compelling performance. What makes a piece "play well" isn't necessarily that it be "easy"; more that it be natural and idiomatic for the instrument... which will in turn be easy.

If you write a piece that a player struggles to play, that will sometimes come through via a muddled and strained performance. There are famous exceptions that prove the rule on this, such as the extremely high register opening bassoon passage from Rite of Spring through which Stravinsky intended to convey strain, or the trumpet squeals of Ray Nance at the end of many Ellington arrangements.

Generally, though, you want the players to be able to play your piece well. And "err on the easy side" is an effective way to do that, especially when writing for instruments you aren't personally an expert with.

maybe I don't use the words right, but go to each of their wikipedia pages and search for the word "court". You will see that Bach, Mozart, and Beethoven depended much more heavily on the favor of the various noble courts, while Chopin and the later Brahms were more commercial (not in a bad way, just in a historical way)

Chopin was special because he wasn't bounded by the orthodoxy - Chopin was so prodigious as a child and had mostly surpassed his teacher in virtuosity as a mere child. His teacher made the excellent decision that he shouldn't prejudice Chopin with preconceived notions of piano technique so Chopin was free to develop his own way of playing from young through his formative preteen and adolescent years.

Wiki entry on his teacher:

https://en.m.wikipedia.org/wiki/Wojciech_%C5%BBywny#:~:text=....

I think what they're saying is in the times of Mozart, Beethoven etc you would not have been able to survive exclusively or mainly as a composer for piano. A career like Chopin's was only possible later on when music and the arts in general was more established and composers didnt live or die by royal patronage and the odd subscription concert here and there. Chopin is unusual in comparison to those earlier composers but not really unusual for his time. Liszt is another contemporary who wrote mainly for piano and there are probably at least few other lesser known 'specialists'(none of which come to mind alas)