Ryzen 3xxx could have improved I/O EASILY as compared to 1xxx and 2xxx just due to the switch to PCI-E 4.0.
All it would take is a motherboard vendor to take the risk:
Wire the 16 PCIe-4.0 lanes from the processor to a PLX chip and have it split out to four PCI-E X-16 slots. They can configure as X16 4.0 - 0 - x16 4.0 - 0, or x16 3.0 - x16 3.0 - x16 3.0 - x16 3.0 or some other combination with a 200% subscription. Or, if you don't want to do any PLX buffering, and have no oversubscription, have them do 1 X 16 4.0, 2 X x8 4.0, 2 X x16 3.0, 4 X x8 3.0, etc. In MANY MANY cases, there is no real need for full x16 PCI3.0 from configurations that have 4 cards in them. There are precious few Threadrippers out there that have more than 4 cards in them. This also discounts the fact that the X570 chipset itself is also offering 2 X x4 4.0 slots as well.
That's a TON of I/O for a majority of the HEDT market. Yes, absolutely, there are edge cases out there that need even more I/O, and 64 PCI-E lanes coming out of TR is still going to find a few takers. The problem is, can you show me even one TR4 motherboard that has more than one x16 slots that's close enough to the socket to actually run at PCIe-4.0? I've looked at a couple, and it looks like a few of them may not even have the first slot close enough to the socket to run at 4.0. Granted, at this moment, that's not a huge deal, but it will be. If you really want that extra bandwidth to the CPUs, you're going to have to tear up your existing TR4 motherboards and put in at least one PLX chip to drive two slots. And, given the size of those boards, you'll likely need multiple PLX chips to run 3+. Now, granted, the above board that I proposed for AM4 might need more than one PLX chip to drive four slots as well if they can't get the distances to work, but, that'll still be cheaper than doing a TR4 board with more than one PCI-E 4.0 x16 slot.
The only MAJOR remaining advantages for TR4 over Ryzen 3xxx is double the memory bandwidth, which is somewhat hidden by the larger Caches on the Zen2 chips, and the much larger possible number of cores. That's it. Those are still big advantages, so there will be a market for them, but, I just don't see it being quite as pressing in the short term.