It sounds like your main concern is that once your inbox is decrypted by your local device it could be used by Proton to train Scribe or for some other (perhaps nefarious) purpose.
For the first point, I think the technical challenge of creating a distributed machine learning algorithm, which runs locally on each user’s device and then somehow aggregates the results, is much more difficult than downloading and using an existing model like Scribe does currently, but I agree that it is theoretically possible. If Proton ever overcomes that challenge and offers that feature, I hope they handle it as I suggested above for Scribe: an option to disable it the first time you use it. As long as I could disable it, I would consider the risk minimal. As it stands today, I consider the risk negligible.
For the second point, it’s true Proton could program their app (or their website) to send your decrypted inbox elsewhere. (That’s true of every email provider, unless sender and receiver have exchanged PGP keys, since email is a plaintext protocol.) I trust that they don’t, based on my assessment of the available info, including discussions like this. I certainly consider them much more trustworthy than Facebook/Meta.
As a general point, I think a lot of security/privacy for services like Proton comes down to trust. It’s important to keep Proton honest and to keep ourselves informed. I’m glad we have communities like this to help us do that.
No joke. This belongs in an exhibition.