Yet Nikon and Sony have brought to market professional-level cameras capable of non time-limited 8K, and that have retained IBIS. If they can do so, why can't Canon?
The only conclusion I can reach is deliberate market-segmentation. They have decided to send out a clear message (with which I have no argument) that stills shooters and casual videographers should still go for the R5, and that the R5C is not in any way aimed at them.
I'm certain Canon could have brought IBIS to R5C, they chose not to do so. Here's how I see this whole R5 pickle happening.
Canon developed what they thought was a revolutionary hybrid camera that could shoot 8K video - R5. It was never meant to have unlimited recordings. It was meant to give a sub-segment of photographers who also do video a taste of Canon video prowess and lure them into Cinema line.
What Canon did not anticipate was a significant backlash over overheating of the R5. Obviously blown out of proportion like anything on the web. Sony A7SIII comparison tests were humiliating for Canon even though you are comparing apples and oranges. Video camera 12mp vs primary stills 45mp.
Canon may not known about A7SIII specs or perhaps they didn't care before the release. What they certainly didn't anticipate were the side by side comparisons with Sony. Perhaps, at this moment, someone at Canon got enough justification for a new product (R5C) based on a segment of people who, as it turns out, wanted unlimited video recording in a mirrorless format. And so R5C project was greenlit internally.
Canon set an aggressive, for hardware at least, timeline to launch a modified R5 with a fan this time.
But they simple didn't want to invest the engineering cost to have active cooling work with IBIS. It all comes down to this. Canon spent a ton of R&D and development money to make R5 chassis. They want to extend those cost on another product, not engineer one from scratch.
IBIS working with active cooling required, and this is my guess, a complete or major redesign. That's huge cost, that's pushed launch date, etc, etc.
So, What Canon execs do? They, correctly, say we can't add IBIS into R5C. And they are correct. What of course they wouldn't say, is that would actually decided not to redesign the whole body to make it happen. It's a simple business decision. Rational, logical.
Why did Canon leave micro-HDMI? Again, back to chassis engineering. They wanted to lower the cost of R5C development as much as possible.
Will they introduce a better model in the future? Nobody knows.
Nikon Z9, Sony A1 8K + IBIS - those guys had it planned and designed from day 1. Canon R5 was designed for short bursts of video.
That's my take on the situation.