Air cooling might have plateued (yet there have been fan improvements the last decade). But so has watercooling as well. At this point I no longer see any point with AIO watercoolers due to noise, long-term reliability and sub-optimal mounting options. First off, watercooling only makes sense for extreme overclocking, it's not longer possible to get a >30% gain easily without extreme cooling and overvolting, since both CPUs and GPUs have already taken most of this cheap gain through their boosting. Nowadays I see people boasting about overclocking a few hundred MHz on a >4 GHz CPU, so at this point it's just pointless for any real usage, unless you're doing it for sports. And of course, bumping voltage will kill the CPU over time.
My largest problem with AIO watercoolers is that it prevents you from doing cooling properly. You are usually limited to mounting the radiator at the intake or the exhaust of the system. Putting it at the intake will make the CPU cooler, but put all the heat back into the system, which is stupid. To make matters worse, many builders seem to favor low-airflow cases these days, which makes the AIO watercooling just ridiculous. Putting it at the exhaust will make CPU cooling less efficient but be better for the rest of the system; GPU, VRM, harddrivers etc. But to do watercooling properly you need a case which puts the radiator in a shaft of airflow which doesn't go through the rest of the system, which only a couple of cases do.
Everyone knows watercooling loops gets full of corrosion and various gunk. After 5+ years that AIO cooler is no longer going to perform like in the beginning. And pumps generally becomes more and more noisy over time.
With overclocking becoming less and less relevant, I no longer see the point these AIO watercoolers. Just get a case with decent airflow, a good CPU cooler with a decent cooling area and a single good CPU fan. CPU fans might be good enough at this point, but I still don't understand why the AIB makers of graphics cards can't utilize similar cooling (yet they manage to push out 30 variants of the same GPU :facepalm: ) I don't care if it takes up 10 brackets in the back of the case, I want one of these on a GPU!