So what if. you couldnt destroy a planet either by orbital bombing or landing troops and then lifting all the population?
Instead you occupy a planet, for a limited time frame, after that time frame the planet would revert to original owner.
So what would be the point of occupying a planet? well first of all you get to use it like you own, with a small difference, you cant destroy original owners buildings.It is exempt from corruption so wont add to you corruption total for organics, or it wont add to you total planet cap for toasters. For your efforts you gain all benifits of planet any science points, income or the ability to remove the resi from the surface.
Not rendering orbital bombing useless here it would have to change slightly to adapt, but No one could ever be destroyed and kicked out of the game. True in real life its different but this is a game, and you loose everything here and you probaly move onto a different game. This method you kind of sulk for a given period whilst planet is in enemy hands, but once occupation period is over you get it back, it hasnt destroyed you game and people are more likely to keep playing