I think you misunderstood. Them making money trough straight payments AND through ad revenue are both completely fine incomes.
However, there is no morality in denying them both while still benefiting from their goods and services. You'd support my argument if it was about some local busines. For some odd reason this shifts peoples perspecives. Someone offers something and says "hey it's not for free, but I won't actually know if you paid or not" (well YouTube does know, but that's secondary).. It's not right to deny them their pay. There are no consequences to it, but you know that it's not sustainable if everybody thinks like you.
As of now, Lemmy is still quite niche. People wouldn't generate a high, stable income on Lemmy sized platforms. It's fine if it's just for fun, but it's not really viable as a full business.
Streaming platforms that compete with YT conform to most exactly the same conditions. They need some form of income.