Why bother? You're going to have a chipset anyway. Why not just expand it's functionality? AMD is after money, just like any other business that's not a non-profit. They are reserving the Threadripper for the higher end SKUs for now, but there is still a market for HEDT, just a small one. Why not go after that market with a high end chipset? It is doubtful that the boards will cost that much different than the existing TR4 boards do, at the very least due to having to route all those PCIe lanes. It will be LESS expensive to route memory channels because there will be just two of them (yes, yes, DDR5 is two sub channels each...). It will be less expensive because the VRM and power routing of the board will be simpler around the processor due to AM4/AM5 drawing a lot less. It will be more expensive due to the more expensive chipset. I think that, at worst, it's a wash.
And, yes, there certainly is the possibility of bottlenecking at the uplink for the chipset. This is why I propose that they use the x16 PCIe 4.0 link from the CPU for the uplink. That's a TON of bandwidth, and would only start to really be a concern with 3+ x16 PCIe 3.0 cards that are operating at near 100% bus utilization. We know that that's an astonishingly rare situation, unless you're intentionally running a whole bunch of 4 X M.2 NVME cards, fully populated, with RAID-0 on all drives, at maximum utilization on the regular, and if you're doing that, why are you only using an HEDT board in the first place?!?! Using the x16 as the uplink leaves the old x4 uplink free to drive an m.2 NVME port, and you still have the one that's always been there to drive a second one! Plus, AM5 seems to add the possibility of an additional x4 link for some SKUs.
The foundation of an HEDT platform is there. It's up to AMD to choose to use it.