You need unlimited 8k long take recording and IBIS? Really? 4k recording with IBIS isn't enough? Short run 8k recording with IBIS isn't enough? What are you filming that you absolutely need unlimited 8k long takes...but also IBIS...so neither the R5 nor the R5c meet your needs? And you need this so bad...you're still using a 5D mark III with 1080p? You need unlimited 8k with IBIS for your films but you've been getting by, for years, on 1080p with no IBIS?
When the R5/R6 first hit I was one of the people who was very upset over the thermal limits because they made the bodies practically unusable for filming of any kind. Turns out that was bad firmware and Canon fixed it rather quickly. They still have thermal limits, but they aren't that much worse than something like a Fuji X-T4. For the way most films are captured/produced, they work just fine. And if you really need long takes, you can always add an external recorder which you would probably want to do any way for long takes.
The R5c trades IBIS for no thermal limits. They almost certainly had to make that trade off. You've got to get the heat from the sensor to a surface the fan can move air against, and that's a bit difficult when the sensor only has physical contact at four corners. This isn't "cripple hammer", this isn't Canon making a mistake (like firmware 1.0b1) or trying to make your life difficult. It's thermodynamics. And no, I don't want to hear about the A7s III. 12mp readout != 45mp readout. Same thing for a "locked sensor" mode. Even if Canon had that tech ready, it's not as good at transferring heat as you might imagine. You can't apply thermal paste every time the sensor IBIS is locked, nor clean it up when unlocked. And if that's how things worked, you still wouldn't have IBIS in some of the video modes.
The people who actually "need" what this camera can deliver don't care about IBIS. They're using gimbals or any number of stabilizing rigs. This is a camera for professional 8k cinema production. It is not a vlogging camera. It's not even really a hybrid camera. Canon has three of those already.
If you're still shooting a 5D mark III because it meets your needs, fine. It's still a fantastic stills camera. If you're passing up the R5 because you think you need unlimited 8k, and IBIS, and weather sealing...then you're kidding yourself and missing out as a result. If this camera not having IBIS or having stills compromises (i.e. dual-boot) upsets you, then you were never the target market for this camera. You should be looking at the R3, R5, or R6, with or without an external recorder depending on your actual video needs.