When you start making your open source project worse for your users because you are not making enough out of it I'll choose to use something else.

There's a point where it's too much and it just feels like a trojan horse when later you stop caring for your free users.

The answer to "how should free things make money" is to not make them free. Any counterexamples are very fortunate. I don't know why people insist on giving away things for free while they actually desire to make money from those things. If the thing is valuable enough, someone will pay for it. Else...not