There used to be a "big" market for aftermarket GPU coolers, is that not the case anymore? Arctic Cooling used to make a myriad of coolers such as the Accelero Xtreme GTX Pro / Accelero S1 / NV Silencer, etc. Have users or builders become that lazy where swapping out OEM cooling with 3rd party options is unheard of? Maybe it's the manufacturers who are to blame with threats of voided warranties?? Whatever is the reason, and I'm going to sound like an old man at this point, I miss the good ol' days.
Nowadays you only have the Accelero Extreme and the Raijintek Morpheus. Unfortunately they both have very large clearings for RAM/VRM etc, so installing one makes the card 3 slot large without fans.
That's the smaller problem. The bigger one is that fitting VRM and RAM coolers is much more difficult. Due to the increased heat output of these, simple thermal glued heatsinks might not do it any more.
And the biggest problem is that they keep changing the card mounting positions, which makes the aftermarket heatsinks completely incompatible. Even if they have some sort of universal mounting plates, things like the new Geforce cards putting the RAM so close to the GPU will make the heatpipes getting in the way of the RAM heatsinks. AMD also changed mounting holes with the Vega and again with the RX7000 chiplet cards.
Also I don't know if even the Raijintek Morpheus can handle a 600W rtx 4080.
On the other hand, you can get watercooling plates for damn near every card type nowadays, including full AIOs. They are more useful because they also reduce the card size from 3-4 slot monsters to 1-2 slot ones, and they are more likely to be able to cool the modern absurd cards. Alphacool even offers a service where they can make new plates for latest type of videocards if you send the card to them.
I should mention though that sometimes the new cards also use existing mounting dimensions. The HD6950, RX480, and RX6600 all have the same dimensions for their mounting holes, so the aftermarket Scythe cooler I used on my HD6950 also worked perfectly on a RX480, and presumably would work fine on the RX6600 too (didn't test that one yet but the hole positions match up when comparing pcb pictures).
Ultimately the cards themselves already come with fat enough heatsinks/enough heatpipes, that aftermarket coolers are not needed. Deshrouding and replacing those whiny 90x12mm fans with a proper 120x25mm one seems to help a hell of a lot more.