Unlike proton mail , microsoft offers basic IMAP POP functionality of its desktop app for free, Maybe proton should offer the same "essential" email functionality for free before criticizing Microsoft. there are many ways to monetize a service without rendering the free version legless.
Proton encrypts and decrypts your data on your machine. The secure key for this lives on your machine and never leaves. Proton do not have a copy of your key because if that key is shared with anyone, human or program, then it is no longer secure. In order to build the feature you're talking about, that security would have to be broken. Not changed: broken. Made ineffective. Thus defying the entire point of the product.
just let me encrypt my data locally. I don't trust their obfuscated JavaScript to handle my encryption keys. Give me IMAP and I'll use my good old client with my OpenPGP plugin.
Your data is encrypted locally with Proton. Your second sentence is what you really mean, and I'm not saying you have to use or trust Proton, just that because of that local encryption of the data, third party apps can't access the data without compromising the security of the service.
Your described setup takes knowledge (and patience!) which customers of Proton do not possess. If you do, Proton is not the product for you, but it doesn't matter because you can build and maintain what you need.