It's more they probably have partnerships and they used to have ads on their website. I can't give an estimated figure but 5k doesn't seem like a sustainable figure in comparison to how expensive server hosting can be.
People fail to understand how much money it costs to host servers for Minehut especially since their ascend in growth. The system ongoing right now is still far better than some of them in the past.
I don't think the plans are going to change anytime soon, I also believe it's quite reasonable. MH20 is unlimited plugins and basically is free (if you consistently vote) which is a huge improvement from prior systems.
I've never used the plugin before but maybe try doing /save-all every once in awhile.
If you could put "not saving" it a bit more detail I could try offer a better soloution.
This is an unfortunate occurrence. In future before you log off you can do /save-all which will save the progress without it resetting any progress, if you have the plugin "CoreProtect" you're able to go back to a previous save.
Check if they're properly installed. If you click out of a plugin after hitting the install button before it actually shows the tick then it wont install.