"Undue burden" and good UX is a wide chasm. In the last few months of my last role we went from the 1 account to multiple separating environments and the additional cognitive load and extra work migrating was not trivial (plus trying to keep costs down duplicating things for migration).

I can see how starting with a pattern of "account per X" would create intuitive boundaries. When you say "per service" what kind of service do you mean? Business related web service API? AWS product? Other? Interested in what boundary line made sense for you given the large number of accounts you say you're happy with using.