Sorry, but I’m not seeing the issue. Proton is a business and it charges for its services.
Their pricing plan and features are quite clear to me?
I think I’m a little lost with what you’re proposing. Are you saying you think Proton should still provide an @pm.me address on downgrading to a free plan? That feature isn’t available in the free plan?
It might be best to find an alternative e-mail provider who can provide what you need? I can’t see where Proton have gone wrong here.