The rest spamming issue would be addressed by some type of limit on resting, either time or resource based.
-3 short rests per long rest
-long rest costs X food or is only available every X in-game hours or might result in a random encounter or will risk failure of some quests. There's lots of different ways to do it.
Your suggestion of food adding slow regeneration: this would address the issue of eating food during combat, but be worse for out-of combat. Why make us wait for healing? Especially since we can short/long rest and full heal with the click of a button.
I think it would be simpler to just prevent the consumption of food during combat.
We don't yet know if Larian will change the limits, but the part of problem with them is that some classes restore abilities as well.
About the change for food: that's attempt to turn food in nothing more than a weak option for dealing with only very small hp-loss. So it'll still help a bit, but no one would ever think about healing up from 5/50hp to full 50hp with food.
