That's a lot of assumptions.
We don't know if it would be too expensive to keep the EF versions of these lenses alive. From what I understand there is a lot of hand assembly and hand finishing with these supertelephotos. Those are not things that would be impacted by keeping both lines alive. Most likely with these super expensive lenses Canon makes a set number and then switches to another model or line until inventory drops and then builds more. We don't have any idea how involved it is to switch from EF to R mounts. Keeping both an RF and an EF version in the catalog may not be nearly as problematic as you imagine. Would they make more EF versions once current inventory runs out? That likely depends on the demand not the cost.
While SLRs may be more complex than Mirrorless, we don't know if that makes them significantly more expensive to produce. Canon has been making SLRs for nearly 100 years. They have the manufacturing down to maximum efficiency. Given that they can offer DSLRs for $500 the cost difference between a DSLR and a Mirrorless camera may be insignificant for all we know. In addition, it's not as though there are no commonalities between DSLRs and Mirrorless cameras. They are actually more alike than they are different and no doubt many costs can be shared by both lines.
A shrinking market does not automatically mean a consolidation of lines. It's way too early in the life of Mirrorless cameras to know what percentage of customers will switch to the R line. As I posted before, Canon probably has a magic number in mind of what percentage of customers they can afford to lose vs. what it would cost them to maintain both DSLR and R lines. In many ways, a shrinking market in which each existing customer becomes more valuable to the company would argue for keeping both DSLRs and Rs going.
