No.
- 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
- doesn't work for emailing someone you have no say you want to send an email to... Idk a youtuber (first example I could think of where you know you want to talk to them but you have no other means to do so). They have their email published. Now what? You can't email them asking for their phone number so that you can exchange email passwords because they won't give it to you, and that exchange is happening unencrypted
- if I have a way to contact someone over signal, I'd rather use that than email