Is that why it's usually used for cryptocoins?
Basically, they still ultimately rely on trust, just like others. While they're drastically different from e.g. Gmail, trust requirement is still there.
If one's well-being really depends on security, it's best to err on side of caution. Meaning, something like MailPile on the client side, very cautious non-automated key exchange, and, optionally, self-hosted email with at-rest encryption (e.g. Postfix+Dovecot+GnuPG pipe) is probably much better option than anything Protonmail can offer in their current state. Or avoiding email (as it's not yet there and probably never will be) and using something like Wire or BitMessage or whatever (I'm not a security expert) has both client-side static and auditable source code and limits metadata leakage.
Yes there is of course trust involved as with anything. Still, for the majority of the folks it's better to trust someone else with their mail over running their own servers with gaping security holes. Avoiding email is not really possible yet and in the very near future...
Well the end it's only email. You decide whom to trust with.
Personally, I feel better with Protonmail. It's not better nor cheaper than gsuite it just has a totally different proposition as a product.
It's an email service. IMAP is a standard, not some feature they are charitably implementing.
If you're concerned about the authenticator app being compromised then a dedicated phone or hardware token device can be configured.
As I said,u2f has better security but at least financially speaking,time based OTP has a lower barrier of entry while being superior in it's security to most non-u2f alternates (phone call,sms,etc...).
I don't think lack of u2f alone disqualifies it from being considered as having top tier authentication.