I'm not a LL, but honestly this doesn't appear to be a sensible longterm (or short term) business decision to me OP. Not a LL, but I own and operate 7 businesses. Will focus on that side of things, rather than the moral aspects of it.
Long term - generally it's always better the devil you know. They're good tenants, they're making 80% in the midst of a pandemic, without you really having to invest anything or make much effort...
What's your acquisition cost on installing a new tenant? How many months will it take for you to reach breakeven on that? IME, acquiring any new customers is ultimately a lot more expensive than retaining your existing ones. You've already covered these costs with your existing tenants, so your 80% is almost pure profit (I'm not including capital gains/movement). Unless you can quietly line up new tenants in the background, ready to move in and save you from any void months, then kicking out tenants currently paying 80% seems a bit short sighted.
Short term - the costs of relisting and acquiring a new tenant might conflict with your cash position, which sounds precarious from what you've said. You're feeling the 20% reduction and you've felt the need for a mortgage holiday, so for me the costs attached to relisting would be jumping from the frying pan into the fire. Can you realistically afford any void months?
There is of course the risk that they'll choose to leave anyway, as they probably can try and downsize/cut their costs, so if you're preempting that, then it makes sense to end the lease on your terms. This - or having new (better) tenants ready and waiting - would be the only scenarios that make sense to me. Other than that, it's just a case of principle and you believing they should be paying 100%, which is completely fine, just maybe not entirely realistic with everything that's happening in the economy at the moment.