As it is the longevity of intel boards are pretty short lived given that they only really support two generations of cpus (three if you count the interim refresh model) with little performance improvement from one to the next. It's already very close to buying a new board for each cpu.
If they made it to where the bios couldn't be updated to support the following gen it would likely hurt them pretty bad. For instance z87 working with chips intended for z97 with a bios update. If this didn't continue with z170/270 it would literally mean buying an entirely new board for each cpu gen upgrade.
That would further hurt sales I would think. For those who might upgrade from an i3 or i5 and choose to go with the newest i7 one gen ahead, they may decide the requirement of a new mobo wouldn't be worth it (buying a new gen cpu already isn't proving to be worth it unless it's a model upgrade not just a generational upgrade).