My existing SD cards for my R5 would have hit buffer limits pretty quickly. I didn't even understand the V ratings when I bought them for my 5Div so got the wrong ones back then. The R5 forced me to buy new UHS-II and Type B cards to make sure the functionality worked. Yes, I could still buy a low spec SD card and take single shots if necessary but dual slots gives you redundancy anyway in case of failure.Uh, no it won't...you will be able to use your existing SD cards there.
Buying a camera with a CF Express slot (because Canon took your advice) WILL force you to buy new media even if regular SD speeds are satisfactory, if you have been using SD all along.
You are in the position of advocating forcing everyone else to upgrade to what you like (and never mind the fact that the reasons you give for liking it have largely been debunked by other replies). I generally agree with the things you say but this time, I really hope you don't get what you want, here.
The new cards were a significant cost and I wanted to make sure they would work for a long time. For my 5Diii I had to buy CF cards so it is not unusual to force users to buy new media.
In hindsight, I wish that Canon used dual CFe Cards in the R5. The space is there, the cost and availability at the time was the same and it is better to have dual media IMHO.