Eh, even you were only allowed to pick usernames by splicing together 1-3 english words ([url=https://www.oxforddictionaries.com/words/how-many-words-are-there-in-the-english-language]of which there's apparently about 180000~750000, but let's be conservative[/url]), you'd still get a ludicrous 180000 + 180000^2 + 180000^3 = 5832032400180000 (about 5.8*10^15) possible usernames, ignoring limits on total character length and whatnot. Judging by how many people use this scheme, I suppose it's a fair enough estimate of how many legible usernames there are.

Which at least indicates that the future probably isn't going to need highly butchered and concise usernames. But it won't stop people from using them anyways, I'd guess.