>>> Making members pay is a dealbreaker for me.
>>> It seems like a member-pay model affects every member, and therefore, affects all groups, even if indirectly.
I think I now get what you mean, so if I interpret things correctly:
(1) A plan (Jan-2021 or similar) which caps the number of (free) users in a group, and requires payment from the group owner for more membership capacity, is technically "making [Groups.io] members pay", because when the new non-legacy group now uses up their free capacity, no new members can join the group unless the group owner either pays for a free membership capacity increase, for a group upgrade, or prunes membership, which itself can result in unintended removals making (other) members "payforit" indirectly.
(2) Samuel's idea piggybacks on (1) and adds an extra payment feature; instead of only the owner having to pay in order to increase capacity (which would still be an option), Group.io users can now themselves purchase an optional Groups.io sitewide account upgrade (VIP pass if you will) that will allow them to bypass the free member/slot limit of any group and join it anytime. But while "optional", that account upgrade could still be considered "making members pay" if looked at from the same perspective as above, because if the member doesn't purchase that voluntary/optional account upgrade (or the owner doesn't pay) the member still cannot join said new non-legacy group they like because it just so happens to have no more free capacity.
So yes, in both of these cases, with cap-limiting plans like these, at the end of the day it is technically making members pay one way or another if you look at it from the perspective of any Groups.io member wanting to join a new non-legacy group and not being able to without SOMEONE having to pay something, voluntarily (member VIP) or involuntarily (owner increases capacity); but from other perspectives as well it does seem there are other indirect ways by which members pay one way or another.
That's partly another reason I'm not personally fond of cap-limiting per-member-pay plans; just like in a restaurant buffet, I'm not fond of limiting user options in order to generate income, I think enhancing/offering more user options instead is better, especially for the long run. Or a mixture of the two if absolutely necessary, but still much more enhancing than limiting.