What choice is that? I choose my subclass and my ENTIRE ability set is decided. Thats literally 1 decision. I cannot change course with my class partway through, because there is no offramp, no new choices. Only an ASL which again is a false choice, because one option is usually VERY strong compared to rest.
Even if you coun the ASL, its rare for a game to last more than 10 levels, so in the ENTIRE campaign you make literally 3 choices for your character - subclass, and 2 ASLs. Thats not fun, thats boring as crap.
THIS is why people multiclass alot, because thats the only functional way to develop your character. Even in the rare game that goes to high level, its rare to have a single class because there are literally no options. Every open fist monk or whatever plays the same (unless you make BAD asl decisions, then your just worse). The only way to not be identical to literally every other matching subclass is to multiclass.