> TSMC's 7nm process is stable, providing excellent yields and has plenty of available capacity
Yields in terms of getting lots of operational dies, sure, but one of the underlying problems here is chip quality. They have lots of chiplets but most of those chiplets are not fast enough to hit the clocks advertised for the 3900X.
In fact, even a lot of the chiplets sold as 3900X are not fast enough to hit the clocks advertised for 3900X, as well as elsewhere throughout their range. There are a lot of people finding their chips only boost to 50-200 MHz less than the advertised frequency of the chip.
Essentially, the boost algorithm now takes chip quality into account when determining how high it will boost. And most of the chips have silicon quality that is too poor to hit the advertised clocks, even on a single core, even under ideal cooling, etc etc.
Thus, AMD has the somewhat dubious honor of being the first company to make the silicon lottery apply not just to overclocking, but to their stock clocks as well. They really wasted no time before shifting to anti-consumer bullshit of their own; all they had to do was advertise the chips as being 200 MHz lower and everyone would have been happy, but they wanted to advertise clocks the chips couldn't hit.
And again, the underlying problem is chip quality - a lot of these chiplets can't boost to 4.3 or 4.4 GHz let alone 4.7. AMD simply can't yield enough 4.7 GHz chiplets to go around, even if the chiplets are nominally functional. The process may be "stable and providing excellent yields" but it's not stable and well yielding enough to meet AMD's expectations.
That's a major reason they're now introducing 3700 and 3900 non-X variations - that will allow them to reduce clocks and satisfy demand a bit better.