I don't have a view about whether Canon is likely to start keeping older models on sale after a newer version is released (although as you say, Canon hasn't been in the habit of doing that), and I don't expect to see an R6 for an RP price any time soon. However, there are a couple of points in your post where I disagree (FWIW!
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First, I would be very surprised if camera gear, at least anything above the very cheapest entry level models, are priced on cost. I am confident they are priced primarily on demand, ie what will the market pay. So I doubt the fact that cameras don't beceome cheaper to manufacture just because a new model is released is particulary important in itself. Canon is simply trying to sell camera gear for as much money as it can, so ideally it will push consumers towards higher profit models (generally, higher end and/or newer models, I expect). If you assume a new model will command a price premium and hence will be more profitable, keeping an old model around may not be a good idea (assuming a sale is less profitable) if it means too many buyers opt for the old one. Equally though, assuming the newer model really is better than the old one and if the cameras can be in two different price classes (eg what Sony has done with the A7IV relative to the A7III), keeping the old model on the market at a relatively low price may make a lot of sense to target different groups of buyers without going to the trouble of developing another model.
Second, I think it is a fallacy to think Canon needs or necessarily wants to recover the costs of a model before introducing a new model. Canon wants to sell camera gear as profitably as it can. If that means retiring a model and replacing it, so be it. (The statements you so often see on the internet where someone claims Canon (or some other manufacturer) is not doing something because they don't want to cannibalise sales of their other products drive me crazy. Yes, manufacturer's engage in product differentiation to try to increase their overall profitability, but primarily a manufacturer cares that you buy one of its products rather than someone else's product, and whether you buy the manufacturer's product X or product Y is much less significant. Further, I expect many costs are shared across multiple models to some degree or another, so I doubt it is easy to allocate all costs to a model specifically in any event. For example, I am sure a lot of R&D goes into AF systems and the AF system which ends up in a particular cameras is both an evolution of the AF system in earlier cameras and part of the development process which will lead to the AF system in future cameras. I think you can see that quite clearly in the mirrorless era, where newer model cameras (eg Canon R7, Sony A7IV) inherit AF systems similar to those in much higher end cameras (eg Canon R3, Sony A1). AF functions such as eye-tracking are heavily software dependent, so (I expect) it is cheaper for a manufacturer to have one code base and deploy it in all cameras, rather than developing separate sofware just because cameras are in different classes in other respects. And, of course, that doesn't mean the lower end cameras will necessarily have the same AF performance as a higher end camera, given that inevitably there will be hardware differences. As someone else (Neuro, I think from memory) has pointed out in a recent post on CR, the R3 has a sensor with a higher readout speed than the sensor in an R7, which allows the R3's AF system to receive information more quickly, which at least theoretically should allow the R3's AF system to perform better than the AF system on an R7, despite other similarities between the AF systems on the two cameras.