I hate to advocate for it, but AWS Cognito is
laughably cheap and certainly not going anywhere. Big downside is that there isn't a way to export; you'd have to list all usernames and have every user reset their password.
Not really helping with vendor dependency, unfortunately.
It takes a lot of sorting through terrible docs to understand, but user pools are OpenID Connect compliant. You get canned login/mfa/forgot ui's out of the box for every platform, and it's so cheap.
50k MAU with Cognito: free. [1]
50k MAU with Auth0: $850/mo. [2]
[1] https://aws.amazon.com/cognito/pricing/
[2] https://auth0.com/pricing
EDIT: Thinking about this post a bit, it doesn't really address your key concern of vendor lock-in and feels like a shill. My immediate gut reaction was "who cares if you're locked in, no way can you beat the price," and that isn't very constructive. I know on an intellectual level that history has taught us a lot of things about monopolies and entrenched players, but emotionally the visible price tag elicits a strong response. Maybe someday Teddy Roosevelt will rise from his grave and save us all.