Free and open instances will, over time, shut down because they’re obviously unsustainable, so they won’t be sustained.
How many of the 5.5k users from lemm.ee are going to say "Lesson learned. If I want an instance that is sustainable I should look for a professional instance or run my own"? I'm not going to say zero, but I really doubt it's going to be "more than 3".
The problem here is that while individual instances may die, there is always a new ~~sucker~~ enthusiast coming up thinking "my server will be different".
But that can not be the only solution. My university offered email accounts for every student. In 1999 this was a very big deal because the commercial services were super limited - Yahoo! Mail offered 2MB, IIRC. But the account was only available while you were an student.