Holy shit, motherboards with more than a single type-C connector. It only took 5 generations since X370 for this to happen.
Because ASM4242 consumes four of those lanes and AMD, in their infinite wisdom, decided to (a) tie it to the CPU instead of the chipset, so that it consumes valuable PCIe 5.0 lanes even though it only needs PCIe 4.0 (b) release Zen 4 crippled in terms of PCIe lane functionality and stick to the same socket for 3 generations, meaning they have no leeway to increase that lane count (c) not upgrade the chipset from Zen 4 to Zen 5 either.
This. ^^
But ... it's not the problem of the socket itself. It's a good decision to keep AM5 around for 4-5 years. The main problem is that USB 4.0 support should have been chipset-bound from the beginning. There is literally no progress for chipset since X670(E), it's the same piece of silicon. AMD requires for motherboards equipped with X870(E) chipset to have USB 4.0 support, which has literally nothing to do with chipset whatsoever. Mobo makers actually did not see other chance but to put USB 4.0 on CPU-bound lanes, which sucks badly when more than one CPU-bound M.2 slot is equipped. Best thing is that when PEG gets downgraded from x16 to x8 when 2nd CPU-bound M.2 slot gets populated, 4 CPU lanes get totally wasted (unused). Given how CPU lanes are valuable, this AMD's move with USB 4.0 support is insane. And given the prices of X870(E) boards, I'd say it's also a ripoff (compared to X670(E)).
Let me sum the things up:
X570: PCIe 4.0 x4 CPU & chipset interconnection
X670(E), B650(E): PCIe 4.0 x4 CPU & chipset interconnection
X870(E), B850: PCIe 4.0 x4 CPU & chipset interconnection
That's three generations of AMD chipsets and no progress in terms of CPU & chipset connectivity. Well done, AMD. Even with 3rd iteration of X370, you managed to get X570 improved with PCIe 4.0.
X870(E)/B850 should have had PCIe 5.0 x4 CPU & chipset interconnection and USB 4.0 support should have been implemented chipset-wise with possibility to disable it in BIOS (well, not everybody really needs it).