It can though, they all connect to form 2 fully-closed buildings.
Eh I think it’s obvious Gringotts is coming later, I don’t know anyone who has ultimately been turned off from that, and I’m one of the people most vocal about Gringotts.
If you want to have the kid/adult debate, EVERY Harry Potter set is for kids, of course. Doesn’t mean they don’t appeal to adults either. Basically every set Lego has ever made, barring the highest end $500+ models, are for kids. I don’t think Diagon Alley is going to be any less attractive than the yearly modular set.