What are you talking about? Your wifi router isn't jumping channels unless you have something fairly sophisticated (read auto channel assignment that recalibrates periodically when nobody is connected).

Doesn't most consumer wifi routers do that by now by default ?

I don't know if it hops when nobody is connected but I certainly found mine to hop while I'm connected. It'd hop around until it finds a channel where wifi doesn't work, and then stays there.

I had to force a channel..

Scanning for least congested freq is very common, but it only performed right before upping of AP iface and no else. So sweeping the band with AP is very impractical.

some of the higher end routers have an extra radio specifically for scanning. Ie: see Cisco

https://meraki.cisco.com/product/wi-fi/indoor-access-points/...

This thing will scan channels, as well as perform security operations.

enthusiast (eg. vanilla-openwrt) and enterprise gear won't, but soho and consumer routers totally do, because in a high density, non-cooperative environments (apartment) it has some potential to make it suck less and ppl like and pay for them "smarts".

Right, but you can't frequency hop without disconnecting most clients. Generally speaking most modern clients are connecting on 5GHz anyways; it's just some generic things like wifi scales or legacy devices that might only be in the 2.4 spectrum.