One of the biggest cons of sl compared to addy is for cold sending. If to share the same 1 domain with multiple family members or friends, reverse alias will be troublesome since got to login to the sl panel and manually add it there. So either the account owner need to manually add reverse alias each time its needed or share the login details so others can add the reverse alias themselves. Both aren’t ideal.
Addy reverse alias for cold sending is easier since it can be created via a predefined formula no need to login panel and whatnot.
Monetarily – no, not a good deal (since you don’t gain anything you aren’t already paying for).
If you are a Proton Unlimited subscriber (and plan to stay one), the way I’d assess the value of paying for the lifetime deal in addition to Proton Unlimited is:
Is peace of mind (that if for whatever reason you can’t or don’t pay your annual bill, you won’t lose access to your e-mail aliases or password manager) worth $200 to you? If not, no reason to pay for a lifetime plan. If yes, maybe it’s worth paying for lifetime in addition to unlimited.
Actually this shouldn’t happen anymore since Proton has stated that they will start using the same policy that SimpleLogin uses, so even if you downgrade to the free plan, you would still be able to continue using your existing aliases. I’m not sure if they have done this already though.