It may be sadly true that Apple doesn't update the iMac in 2020. I still can't believe it, though.
I bought the first one in 1986 and till now I have always seen Apple being the first to make the tough choices. Among them: ditching the FD drive, then the CD drive. Now it's time for the HD.
Every time they did it, they kept the legacy models for about a year (or more, in case of the MB Air) to allow for lower budget configurations.
The iMac is Apple's most sold desktop. It's actually, as far as I know, the best selling computer in history.
Keeping it without a Tx chip and with an HD is mostly ridiculous in Apple terms.
On the other side, unless they decide surprisingly to hop on the AMD train, Intel is offering really crap alternatives for an upgrade. The 300W or so of TDP under heavy load of the new 10th gen chips is the worst nemesis for Apple's engineers. They have done miracles in the past, given the chassis they are constrained to work on. However, Apple is not famous for designing unleashed machines.
If we didn't see a MP for years is ALSO because there was no easy way to replace the CPUs and GPUs without having to waste all the R&D spent on the tMP. Remember the "thermal core" with the bottom fan? Where is it today?
Cascade Lake Xeons are more or less close to their predecessors. Therefore, in the very likely case where Apple will just tweak the design a bit while keeping the shape (like they did with the MBP 16"), the IMP is the only machine they can produce.
GPU wise, we will have all the AMD range at their disposal. From the 5500 to the upcoming 5950(?). Delays aside, all the range should be available by H2.
The two depressing scenarios I see most likely to happen are:
1) Apple is waiting for Intel to deliver a 10nm+ (or 7nm?) that doesn't fry eggs and will come out only with an IMP by the end of the year.
2) They are radically changing the design and as said before by others, they will percolate the innovation top to bottom for financial reasons and we will need to wait until 2021 for a mini-led iMac with a similar design to the IMP.
We (almost) all know that if they used Ryzen/Threadripper chips in the iMac/IMP line, they wouldn't have to struggle this much.
However, getting back to my first point, it is ludicrous that Apple will ditch the HD in 2021. They should have done it in 2016, imho.