In all honesty, there are a ton of us tech enthusiasts who have no problem paying 10-20$ per month to run an instance out of our own pockets. We get the ability to subscribe to content we used to use Reddit for, and we can have a few folks hop on with us. Multiply that by a bunch, and add in community funded instances, and we'll be fine.
Gotta consider server costs were only a fraction of Reddit's costs. Salaries are quite pricey, and we have lots of folks volunteering time which will make it all work.