Most of these complaints are genuine (albeit well-known) problems when explained correctly, eg:
We have a poorly optimized rogue, an optimized fighter and a couple of battlefield controllers, and the fighter makes the rogue seem useless in combat.
Power creep from new books makes old APs seem too easy.
Game balance seems to fall apart at high level play.
There is a PC with unhittable AC - any sensible enemy would retreat immediately.
My ultimate boss fights are over anticlimactically in six seconds.
RD is taking a lot of these threads out of context and not fairly judging them for what they are: requests for help in keeping the game challenging from new GMs.
So what if a noob needs help coming up with challenges for a newly leveled-up party? So what if that same noob needs help challenging a powerful fighter? So what if he needs help understanding how to keep the mobile monk engaged and interested?
Pretending all of these threads are made by people who somehow don't understand that a fighter kills things, is just some sort of confirmation bias. You want to think "kids these days" are dummies, so you find "proof" of it and start a thread removed from any real evidence.