I expect to slowly replace Intel Macs from today until June 2021 when they will finish the transition by WWDC 2021.
Prepare to be disappointed. That didn't even happen in the PPC -> x86 transition. And Intel had a full set of CPU products available before Apple even started the transition. Apple does not. They have had nothing that fully completes in the desktop space. That is why the Mac Mini backslid on ports and number of displays supported. Apple is pretty unlikely going to "fix that" in less than 8 months.
For the x86 transition Apple slid the WWDC to August to have their claim victory party. So June? There is not even a "broken analog clock is right twice a day" precedence for that.
Additionally look at the developer transition kit in the two transtions. For Intel could support driver development for PCi-e cards. Apple Silicon transtion. eGPU are absent. macOS 11 can't do anything except Apple GPU because drivers for nothing else. (or other accerlators/processors that normal sit on PCI-e). So that is delayed also.
The rumors of a "half" sized Mac Pro ... which is probably a move to prune features off the Mac Pro to backslid back to what their later SoC is another indication that they don't have a broad SoC line up on the near term horizon.
The only way Apple completely polishes off the transition to Apple Silicon by the end of 2021 is by dragging the whole back to laptop constraints. There is pretty good chance Apple will finish the laptops in 2021. Given that is 70+ % of the Mac business they would have gotten to the high volume status. Doing the last 10% will be much harder for them given their desired to stay tightly coupled to the iPhone and iPad Pro. ( The M1 die is pretty likely going to show up later as a A14X with some features turned on/off. It is approximately the same size as previous A12X and A10X. )
They may opt to have one SKU Intel Mac for people who insist on an Intel Mac like they did with the
last Mac with a Superdrive. Just do not expect it to receive any updates beyond what is being sold today.
There could have been an Intel product that was scheduled for end of 2020 that slid into Jan-March 2021. Pandemic slid many products. There have been impacts.
By April-June 2021, I wouldn't expect anything new. There is a substantive problem with macOS 11 being so behind of PCI-e card driver support though. And the huge disruption that Apple has done to device drivers in general ( going from kernel extension IO-Kit to IO-MMU leveraged evictions from the kernel System Kit. ). That probably won't completely settle done to broad completion until macoS 12. So there is a major software component here also that isn't likely to close before another OS iteration. ( similar to the hype where Apple said they were going to "wipe out" HFS+ in a year and ....... did not. Once got into the pragmatic weeds of Fusion drives , HDD support , etc. of which large number of people had it was much more complicated than rolling out to single drive SSD homogenous environment of iOS devices. )
Why? Because after WWDC 2021 I expect demand for Intel Macs to dwindle to ~1% of all Macs shipped.
Probably, not going to happen in desktop space; nor with a healthy fraction of the MBP 16" space.
The other problem for Apple TSMC doesn't have that kind of 5nm capacity to soak up 6-7% of Intel output without robbing other customers of capacity. Can grumble about Intel's fab problems but they do have large capacity. It is going to be difficult for Apple to show up and order up 2-3 M more dies of 2-3x the size of the M1 and actually get those wafer starts. (and zero impact on the iPhone SoCs. ).
Apple has lots of money to pragmatically buy up exclusive 5nm lock outs for a short period of time but isn't going to last for a very long time. TSMC will bring up more capacity over 2021, but the problem is that there are dozens of folks with deep pockets that want to feed at the 5nm trough.