The 2019 MBP 16" supports four 4k displays or two 6k displays, because it has a proper midrange GPU. I expect that the 2021 model will support at least as many. People using more than two external displays are in such a small minority that requiring one of them to be over HDMI is not a bad trade-off.
Ok, so I guess let's beat this dead horse some more because you're still avoiding giving an answer.
Irrespective of the GPU involved, on Macs from 2018 onwards, the total number of concurrent displays supported via TB3 ports connected to anything (native TB3, native USB-C, DP, HDMI, DVI, VGA, shadow puppet projector, you name it) is one less than the number supported "total" - i.e. one video channel is hard-wired to go out to the HDMI port.
Now, as you say the 2019 MBP16 supports 4x4K displays, or, importantly, 2x5k or 2x6K. If you use 1x 6K, you can still run 2x 4K. So, I do not think its unreasonable to
question if a hypothetical 2021 MacBook Pro with a HDMI port built in, would lose the ability to run 1 of those 4K displays, and potentially, the ability to run two 6K displays simultaneously. I don't
know if that's going to happen, nobody knows anything yet. But it's not some pie in the sky theory - it's literally just looking at other products with the same technology incorporated. Yes, I know the 2019 Mac Pro doesn't seem to exhibit this behaviour - so as I said - I'm asking a question.
So - in a hypothetical scenario where adding HDMI means it needs (for whatever reason Apple chose to make it so in the Intel and m1 Mac minis) a dedicated channel to itself, and that channel is no longer available to TB3 port usage, do you consider that a worthwhile tradeoff?
Or to put it another way: if it meant some users can'r run 2x 6K displays, or 4x 4K displays via the TB3 ports, so you don't have to carry a $12 adapter, would that be a "worthwhile tradeoff?"
What is technically possible is irrelevant. What actually happens is what I'm concerned with. Apple's MPX modules with a HDMI port don't have the same vampiric configuration as the Mini's do. But it's a choice Apple made somewhere along the line. Given that they made that decision for the 2018, and then - even when losing 1 of the 3 total - chose the same setup for the 2020 model, I see no reason to be confident they won't make the same decision for other Macs.