> A couple of years back I switched password managers, I didn't go over 1000 sites and changed all my passwords, my password manager exported a plaintext file and I had it imported in the other after a small transformation step.

And, modulo the "plaintext" part, I think this is a reasonable usecase. It's equivalent to the "backup" case. I transfer an encrypted blob between devices and decrypt it locally is reasonable.

> No they don't and if they did they would also understand not to upload their plaintext credentials.

Except that you have already stated that you have done exactly this, and you claim to know what you're doing!