OLED is easily the best TV tech you can buy today. Burn in is possible but it's rare unless you go out of your way to try make it happen.
I have two LG OLED tvs. been using one for over 2 years now and looks just as good as it did out the box. The other TV has had more use despite not having it as long. I've had it left on news channels for hours and news channels are the worst thing to watch on an OLED due to the amount of static elements on the screen. Still no burn in. My phones have been OLED for years and my Switch is the OLED model. Yet to see any burn in.
I'll keep buying OLED until Micro LED is available for a sane price and if it actually turns out any good. Mini LED is trash but Micro LED is promising.
it's true they will get dimmer but it's pretty much negligible in real world use. RTings ran a bunch of OLEDs over 5 years (almost 10,500 hours) and you can see how brightness changed:
and the color gamut hardly changed