It's going to require new hardware to support the new frequency so it only makes sense that if you support the frequency it should be required you do it with the minimum protocol version being the one that was around at the time the frequency was introduced.

And yes, tri band routers will be popular as well as switchable dual band routers (i.e. you can flip between 2.4/5 or 5/6 but not both at the same time)