Don’t a lot of people keep separate Twitter accounts for different interests/identities too?

To me it seems like the main question is the amount of friction encountered in switching accounts in clients. In more polished clients like Ivory this is just as easy as it is with Twitter.

I had/have multiple Twitter IDs: one for me personally, one for a particular topic, and one for brands and even other for IoT experiments. One of the brand accounts was handed over along with other assets when company IPR was sold: it was a role account,

The first two I have replicated on Mastodon. I have stopped using all the Twitter accounts, so I have now 'slimmed down'.

No it's not. If what you want out of a Twitter-alike is to be able to decide what the administrative environment is of where your account lives, this is a great answer.

If you don't care about that, then _you don't have to do those things._

I don't have an answer for the people who want to leave Twitter but refuse to touch anything that isn't a perfect clone with complete feature parity and a comparable user base, but plenty of people - even "non-technical" people, manage Mastodon just fine as an alternative. It really isn't that difficult.