Always the same cycle.

1/ New great product is built. People love it.

2/ Once enough people use it, start monetizing in shady ways, annoying users just not too much or they leave.

3/ Very annoyed users switch to another product back to 1/

Small correction:

1/ New great _free_ product is built. People love it.

Image and file hosting services and messengers are the best examples.

I swear it's because the well has been poisoned and it's just impossible to monetize these services in a moral way.

This is absolutely not confined to free products. I did security audits for companies and part of that job was giving a go ahead before we allowed people to install software on their devices. Free software behaved much better than paid software by a wide margin.

shouldn't have needed it in the first place. what happened to pointing to a css file.

True. Userscripts and userstyles should be a part of the browser itself. Userstyles were at some point, AFAIR, at least on Firefox.