If you make items that apply the grounded effect, there is nothing stopping mobile champions from using those items (remember we have all of these assassins that build tank).
As for just banning those champions, well we do the same with mobile champions and we have a lot of them. Why couldn't we have a lot of champions with the grounded debuff? What if we reworked Pantheon to give that debuff? Or give a few immobile assassins that ability? Or hell, give it to supports like Sona so it can lead to some better secured kills in bot.
We do both a buffing and a nerfing then we could lead to a wildly unbalanced scale. As much as I think immobile champions should do more damage (example is Karthus) it really boils down to mobile champions doing too much.
Honestly I think this meta is like whenever WoW has a new expansion. Older gear gets wildly outstatted by new gear. Then WoW decided to lower the scaling of stats throughout most of the game (so no need to make items that give 1K strength).
Immobility has become so strong because we don't give an even trade off for it. This is probably due to Riot changing their design philosophy, leaving previous champions in the dust.
I'm not a game designer. I don't know the answers you probably want. I'm just giving my opinion.