To be fair, Proton also has this path as one of the options. With SimpleLogin the user can encrypt mail with their own public key, without trusting Proton to manage the private one. Same for Cloudflare, Addy, and Forwardemail, there are plenty of options. Probably some other features should be considered if the user prefers the 3rd way.
Added: it also makes it possible to combine the 1st or 2nd approach with the 3rd one, though that feels like a weird thing to do, personally.