No, I *don't* get it, to be honest. The difference between 20MP and 17 shouldn't be that big a deal.
But I know that's what you're thinking.
What I also think you don't get is that it's not about the difference between 17MP and 20. It's about the difference between 17MP and 24, or 28, or 32, or whatever alternatives are out there *today*, and not just from Canon. The 7D Mark II is a 2014 camera. If Canon comes out with an "R7" RF APS-C body (and that's still a big "if"), and it only has 20MP, I'll be quite frankly disappointed. Do I *need* higher resolution? Maybe, maybe not. But again, that's my choice. The state of the art in 2021 should be more than 20MP. If the (hypothetical) Canon R7 is otherwise identical to the R6 but with a 28MP APS-C sensor (which is less than the 90D), I'll preorder it on day one.
If I want to transition to mirrorless, I will have to either buy new lenses or adapt my current ones no matter what Canon does. That realization has opened my awareness to other systems, and Canon has competition in APS-C. I really like the Sony a6600. It's got a great AF system, and I'm seeing amazing images from it with the Sony 200-600G lens (which costs a lot less than the Canon 100-500L). Sure, the a6600 has its downsides - the core sensor is at least four years old, the card slot is limited to UHS-I speed - but it's a viable option if I want to stay with APS-C and move to mirrorless, if a Canon R7 never materializes or isn't compelling.
Don't get me wrong, the R5 and R6 are incredible cameras! With that single release, Canon went from an also-ran in mirrorless to the clear leader. It's great that you consider your R5 to be "the best stills camera ever made". Use it in good health!
But if you really believe 17MP is is enough for APS-C users, doesn't it follow that 20MP should be fine for full-frame shooters? But obviously you thought the extra pixels in the R5 justified the additional cost over the R6. That was your choice, and nobody but you can say whether it was right or wrong.
I'm an APS-C shooter. That's my choice. I shoot mostly with teles and rarely wide angle. Even if you were to convince me that 17MP really was enough for the shooting I do, if I got an R5, I would have to manage raw files that are 2.5x as big as I need, just so I can crop away 60% of the image area. That doesn't make sense to me.