The R1 is not really a follow on for the 1dx III, it is a transition platform to new technology direction. Canon released the R5/R6 to establish the beachhead and prove the technology. Even if the bulk buyers are not clamoring for a mirrorless, any company with an eye to the future is already evaluating the mirrorless alternatives. If Nikon releases a flagship mirrorless and Sony updates the A9 III (at least they already have an offering) Canon can't afford to miss the early evaluation cycle. Canon seems to keep things pretty close, so they would not want a lot of evaluation units in the field. My guess is that most of the bulk orders for the EF 400 & 600 III have already been placed and release of an R1 won't have that much impact. I believe there was a rumor that some of the RF mount big whites would be out this year, so that should take care of your concerns about RF lenses. If not, expect a weather sealed RF - EF adaptor to allow a transition.Those are not several things but 6.
R1 was not released as the surveyed bulk buyers and other 1-Series customers from 2017-2019 prefer SLRs.
That may have changed within the last 52 weeks but the question I'd pose is is Canon willing to take the financial lose of Osborning the 1D X Mark III and Series III EF 600/4 & EF 400/2.8?
Now, if Canon released a RF 600/4 & RF 400/2.8 in 2018 instead and R1 in 2020 instead then that would have been better.
I have no doubt that the MILC tech for the R1 has been there for more than 2 years already but what is needed to support a 1-Series body is the appropriate lens and accessories for it. Thus far only the three L f/2.8 zooms are there. No white long fast primes.
Nikon themselves admitted that their transition to MILC was slow. By my estimate by 2 years. Canon was late by 1.
Sony timed it perfectly. Their FE 400/2.8 came out in 2018, FE 600/4 and a9 II came out in 2019.
I don't think Canon will wait until 2024 to release an R1, although it may not start shipping until 2022 based on the factory delays.
I would love to have one for my Namibia trip this year!