Honestly even if he isn’t famous why can’t Wikipedia keep his entry? Why does Wikipedia even have “notable” requirements anyways?
There’s not a storage issue. Wikipedia can literally have billions of articles and still be easy to maintain.
There’s not really a quality issue. Wikipedia is known for not being 100% reliable. But moreover, they have tons of ways to denote “this article needs citations” and “this isn’t a reliable source”. If Wikipedia is concerned about quality, they can have “verified” and “contributed” articles, just like how distros have “stable” and “user-contributed / experimental”.
Spammers and useless content? This is an issue. But this guy is clearly not spam, the proof being any of his official works. I do agree that Wikipedia authors should remove “spammy” entries and entries on complete nobodies and random things, but you shouldn’t need to be in an Oxford journal to not be considered a “nobody”.
Even things which are famous in small towns and 1000-member groups should be on Wikipedia IMO, because most of the stuff is already on there is about as relevant to me or anyone else (which is to say, pretty irrelevant). If you want relevant content, that’s what the search tools and indexing are for.
Wikipedia is supposed to be “the grand encyclopedia” where you can find info on basically anything. There are already tons of Wikipedia articles on obscure people, places, and things. Way more obscure than this composer even if he isn’t truly well-known. Why does “relevance” even matter?