I like the idea very much.
It is manageable to manualy switch grenades, when you play solo, but when you play in group you can not stop fireing, because of greande switching. Game will not pause in group play. All other characters can throw grenades ongoing, when there action skill is active. But I guess that change is to much in game mechanics and UI.
But I guess it has a big change in balancing.
When AutoBear is active is AutoBear throwing grenades and Moze is able to do it?
For balancing it must be prevented. Otherwise you can throw two different grenades at once.
The only thing I could imagine it will work that way is, when AutoBear is active all the time AB will throw a greande it will be decreased from your grenade pool, too.
So it can happen that you are very fast low in grenades. But moze has not so much heal skills. One important skill for healing is vampyr. When Moze is throwing a grenade she will heal 4% max health for each hittet enemy. (for each skill point / max 5).
So, it could be that you are out of grenades, when you need it for heal and maybe the grenade regeneration is not so fast as you need it.
You see, as much I think about it as more problems occur in my head. I am not conviced that this will be realized. But I like your idea.