I have many nerdy friends who have been Linux users for ages. But most of them don't know such a thing as Openwrt exists or have never bothered to give it a try.
It's a very fun piece of software to play with and can be extremely useful for routing traffic.
Wondering why it isn't more popular/widely used.
I know about it, but I prefer Asuswrt Merlin firmware for my routers, because I mainly use ASUS routers (powerful, modern (WiFi 6E etc) , easy to find second-hand models for cheap) and Merlin firmware is very well integrated with the routers and uses the same UI as the stock firmware, but provides additional features like a package manger etc.
In fact I believe ASUS themselves have started to use some of Merlin's patches in their firmware, which goes to show how professional Merlin is.
Yes, the R4 is the next thing, I'm not sure the wifi board is selling already and what's the current state of the software. However I happen to have deployed a bunch of R3 boards (with metal case) with OpenWrt and they work amazingly good.