My primary cameras are 20mp and I have a Canon Pro-2000, printing 16”x24” from 20mp files really isn’t difficult even if you crop. Of course it does help if you get your framing pretty close, but software is so good nowadays and techniques for maximizing what you have are so advanced the only reason I’d like a higher resolution body is for archival purposes And the desire to not fall too far behind in current tech.
You are absolutly right and in fact, I can't hardly explain it rationally - it is mostly a psychological block, because I have got used to those 30mpx of the 5DIV
I also remember my xy years old post on dpreview, where I was upset at Canon, for going from 14 to 18 mpx, or something like that. Because .... I was worried about the high ISO performance back then.
As for R5 vs R6 .... still think, that if our money situation allows it, we will go for the R5. Both cameras are ultimate machines we have been long waiting for (in the mirrorless sense at least) from Canon. If R5 high iso is not worse more than 1/3 - 2/3 of a stop, I regard R5 (with possible future firmware updates) as being a nice machine to have for the 4-5+ years to come, or even more ....
The more tricky situation for us is the lens - RF ones are not going to be usable on our 70D backup, but we might solve it going full mirrorless, using Rp or its coming successor as a backup, as nowadays, 70D is not a solid backup - it is just that so far, no Canon camera we have owned, failed on us, not even once.