I completely understand and agree with their incentives. Those with the public instances will play the winning/losing lottery, losing being not managing the troll automated induced bad content fast enough. I encourage anyone taking on this challenge to first and foremost get some trustworthy non-toxic non-power-tripping moderators around the world for the "follow the sun" management of the instances.
Some of those will exist no matter what (Twitter accounts are unlimited and still sold) but - limited to invite only doesn't need to mean "limited as in scarce" - there's no reason to share an account if you can just invite the person, instead.
I've managed closed Fb group for ten years now. For new members we have a voting system in place. Inviting wouldn't work because if members could invite whoever they choose it will sooner or later lead to having members that not everybody is comfortable with. A friend of my friend may not necessarily be my friend. It'll create friction and lead to all sorts of interpersonal problems and soon enough it's not a peaceful community anymore, people start to block each other or lash out in comments just because their personalities or beliefs clash. I don't believe in invites anymore.
Google Wave and Googl+ also had invite system, it didn't work out well. Gmail is exception to the rule I'd say.
But people WANT the chance of winning the "lottery" as it were, and going viral.
You're not doing that in your small discord or private mastodon.