While it's doubtful you'll see ads in KDE's core applications, it would be possible for distributions that wish to further monetize their work to fork these applications, placing ads in them.
Why, even FOSS needs support. See OBS Studio, Wikipedia etc. Without supports good projects go to waste.
Developers can decide to introduce it in their apps or not. I am sure not every QT developer will adopt this.
People will be able to opt-in, opt-out. I am pretty sure they provide us with an option.
Most people do not donate, so an additional income thing could help.
The other option would be crypto.
We are not talking about MS who introduce ads in Explorer which need some ad-blocking, hosts or registry hacks. Linux is more transparent and there will be options to control this.
Do I like it, nope. But it is better than alternatives to shutdown project because lack of funding or struggling to expand because only few people are willingly to donate.
Sure, I understand that people need money, I just think it's a patch to a bigger issue that's not solving anything, I don't think we should be putting advertisement in the most unimaginable of places there are, it's just sick.