This particular argument is one that's been had before, and suffice to say, an argument can absolutely be made for mechanics through which a planet has a chance of fending off attackers. Just to start with, surface-based anti-orbit weaponry would enforce attrition on an attacking fleet- there are also things that would have crossover from dealing with doomstacks, such as mechanics that encourage multiple smaller fleets operating in different areas instead of doomstacks, or actual attrition/supply line concerns.
That actually does't work very well because in the abstract you end up with one of the following:
1 - If space forces can destroy ground defenses, then you encourage bombarding with the largest fleet possible to kill the ground defenses as quickly as possible.So this changes nothing.
2 - If space forces cannot destroy ground defenses, then you encourage just sending in a massive transport fleet and accepting that some transports will get shot down.
3 - If the mechanics prevent you from landing without bombarding, i.e. you cannot do #2, then all you've added is a space fleet cost to take a planet.
You have not changed the inevitable. Since the player in control of space is in absolute control of when and how to engage the ground force player cannot win all they can do is increase the cost. Additionally in all three of these scenarios you've created an additional thing for the AI to understand to fight properly and all the additional computation that goes with it. And for what? The necessity of a few extra clicks or slightly more careful management of timing.
Again, I understand the appeal but not every game has the design space to support every possible feature. It isn't to say that it is impossible to make a space 4X with deep ground combat. I can't think of any space 4X that a deep, complex ground combat that did not focus heavily on ground combat over space combat, e.g. Emperor of the Fading Suns. All of the big ones, Master of Orion, Endless Space, Distant Worlds, etc. all have some variant of either landing some ground troops or a simple-time-to-capture.