"the price is so much higher for decent RAM beyond 3600"
maybe, but kinda flawed logic imo... we see the same ram working better as zen & am4 improved.
ie. maybe 3600+ ram is dear cos its hard to get higher on current rigs?
It's actually easy to hit >3600 on recent OC-capable Intel platforms, and pretty much any Zen 2. You don't see it often, because the minor gains aren't worth the increase in price. The money would be FAR better sunk into other components, until you're in the very high-end. Then you simply start to run out of other things to improve.
For Zen 2 in particular, the IMC was improved, and now you can set an MCLK : FCLK divider to 2:1. That wasn't even an option for Zen 1 / 1+. But doing so will actually hurt performance in most tasks until you hit some REALLY high speeds, like ~4200+. You can also exceed 3600 even at 1:1 ratio by overclocking the IF, but even then 3733 and 3800 still isn't generally worth it.
So again this is NOT the first Zen to be able to hit those high speeds, and even though they've decoupled the IF from the memory clock, it won't really matter all that much. The gains above 3600 even at 1:1 weren't that great to start with on the CPU side. The area where you would see the most gains is the iGPU, and graphics performance isn't latency sensitive in the first place so even if you had a coupled IF at 2:1 it wouldn't hurt it.
With that being said, a theoretical decoupled IF and high memory clocks might be more interesting for a 16 core Zen 3, for some workloads.