Will people now regret buying RX100?

You'll have more fun with the 5R for sure and the IQ is better without a doubt. But you have to be honest with yourself and ask will you ACTUALLY carry the 5R around? It's a small, lightweight camera but not a pocket camera. It will require either being carried on strap over your neck or shoulder or in some kind of small case.

The RX100 is very much like your S90, though the lens barrel is noticeably larger but far easier to carry than 5R. I know, I have a 5N and the RX100. They're both great.

I use the 5N when I know I'm going to shoot. I use the RX100 as my substitute for images I was shooting with my iphone that I want to do more with.

Hope that helps!