rofl
Old dog here. I have no problems keeping up with the new mechanics. The new mechanics, in reality, require less coordination, the same reflexes, and less skill than before. When I was raiding in Legion and earlier in BfA, I averaged 90% according to Warcraftlogs...so clearly my age isn't a barrier and I clearly perform just fine in these raids.
The reason old dogs like me complain about Retail is because it is pointlessly mechanic heavy and, once you learn it, insanely boring. It doesn't matter what rdps I bring to the current crop of raids; I play each one of them exactly the same, because the mechanics are the same. This is true for every mdps, tank, and healer.
And then when the group progresses from Normal to Heroic, *some* fights add another mechanic. While people like you likely say, "Oh! Exciting!", people like me are rolling their eyes. Yeah, the same thing I did for weeks, I get to do all over again with a minor change here and there. Sorry, but I don't excited about the "gold star" of AOTC and the same gear I already had with slightly better stats.
What really changed is that when I was heavily raiding in places like Karazhan, I used to enjoy each week because I would have a different mix of players, and that actually affected how the encounters played out. Same with most dungeons. I got *sooo* frigging bored with running the same handful of dungeons over and over again...adding an affix rarely changed how it plays out (i.e. tank charge, everyone AoE, some interrupt, rinse and repeat ad nauseum).
As for the OP's question, the answer is that time gates are substantially cheaper than "skill" gates. I put skill in quotes because the game has never been overly hard. There simply was a lot more content to progress through in the original and first 2 expansions. Blizz has been cutting corners more and more with each expansion since Wraith to the point that we are left with a game of repeating the same things over and over again for a chance of getting the same gear we got the first time with slightly better stats.