Both auto-forwarding and auto-reply are paid features, which makes cancelling & switching much more difficult. Gmail is a breeze comparatively. I highly recommend against using their addresses (e.g. protonmail.com, proton.me, pm.me)
Email forwarding is available for everyone with a paid Proton Mail plan.
(source)
I don’t think the OP is saying to switch to gmail. They’re saying if you think switching from gmail is hard, just wait until you have to pay to forward all of your email if you need to move away from Proton mail if they end up being as bad as Google. They’re saying use something else that’s neither gmail nor proton mail.
I actually did switch from GMail so… I feel like I’m a relatively good place to talk about precisely this problem.
It also was very easy because I used my own domain back then. I switched from GMail to Proton and nobody noticed. I didn’t have to change 1 single account or warn 1 single person.
OP is not talking about switch email providers in general but rather a very specific case which is indeed problematic but is not IMHO related to lock-in.