I think in the end most computer users will sacrifice a bit of security for convenience. PGP signature and encryption for email will most likely never take off. Encrypted chat services like Signal seems to have done much better, likely because of how convenient they are to use.
This stores the private key as a password-encrypted file in a specified location on the IMAP server. It already has multiple interoperable implementations, by the looks of things.
As for the bridge, that is exactly what we'd like to do, as I've said before.
And customer support time and developer time devoted to this would cost money and represents an opportunity cost as well and that's a fact, not a point to be conceded.
And like I've said before, what you'd like to do and what you are doing are different. Nothing stops you from open sourcing it today. Put in comments that the APIs you use are not officially supported if you must. Open source it! It should have been open source a year ago! Open source it!
Some people are just like that :)
Great product, Proton team, thanks.