You certainly make a lot of topics about mail - maybe you need a "master email thread"
I don't think anyone can accurately predict the long term business viability of a company. Here's their Wiki:
https://en.wikipedia.org/wiki/ProtonMail
They recently got $2M in funding, they're looking to grow their pay services to sustain and grow the company. If you're that concerned about losing your email provider, choose a solution that's less likely to go under (i.e., something from Google, or MS).
What you should do:
Buy a domain, choose your email solution (which could be Proton), setup a box or alias so that the email you provide to various services can be easily changed on the backend, without having to update dozens of user facing services.
i.e., if you have
dt@mailguys.com and MailGuys goes under, you won't be able to use that any longer, but if you had,
d@t.com pointing to MailGuys, and you decide to switch to MailGals.com, all those systems with your
d@t.com won't be affected.