The "difficulty" here can also be improved significantly in many different ways. E.g. a browser extension that allows auto-filling a form field with webfinger id is all that's needed to build a better flow for a lot of these federated services.

There are ways which would work without extensions too. E.g. a DHT mapping <some fingerprint> -> instance url. It doesn't even need to be perfect. Worst case you need to fill in the correct instance hostname.