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 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.