One of the main uses of email is communication with companies. And they won’t have a signal account just to exchange passwords with you
No. Email is just a non-centralized protocol. While not everyone uses it the same way, most normal people never use email to communicate with companies, who are increasingly forcing people to use chatbots anyway. So it's not even a reasonable point to make. Password protected emails are meant to be between people who have an established relationship. If a company needs someone to send them encrypted message, they'll have a platform for that, just like Wikileaks or ProPublica, so you're not making a valid argument about that.
If some Youtuber is someone that does anything privacy-related enough that they should be receiving encrypted emails, their public PGP key should be on their YT profile and you can send them an encrypted message anyway with that. Protocols and methods exist already to accomplish what you're talking about. You need to complain to the Youtuber for not practicing good security and privacy, not to Proton for not creating some mind-reading Diffie-Hellman scenario. Really, do you think that you can just send some random person a message that says "click link to open secret message!" and not expect it to just look like phishing?
If you'd rather use signal, use signal and send them an attachment encrypted with their PGP public key. This isn't hard, I don't even know why you're trying to argue all these weird non-existent edge cases like they're everyday issues.