Male hypergamy and polygyny has been practiced by many cultures historically. They had many problems, but low birth rates wasn’t among them. I don’t think dating apps are a sufficient causal explanation alone for low birth rates

Back then you could have 10 wives and start a population of people named Khan.

Like which cultures?

The second factor is access to birth control, most women just dont want to have children unless in very specific circumstances

Historically they were compelled