Interesting concept. I think there should be something like that in the game. Like everybody else said, you wouldn't just sit there and wait for your demise. You would fight back, launching everything you have against them. And another thing, if this were an actual situation, all goverments would not only fortify the space around a planet, they would also add at least some weapons to the surface.
Yeah, I can see where it's going... However, if I would be the invader and I just secured the orbit around the planet, I would not necessarily fly into the reach of the ground weopons. I would keep my fleet out of reach and take care about any ground threat from there...
Let's choose a different approach: drop ships!
Invade a planet with troopers, which need to be researchable. You need a lot of drop ships to take over a whole planet (depending on the population of a planet. Further research might decrease the amount of drop ships needed to take over a planet). Using drop ships can hand you a planet including intact infrastructure. Thus, the invader has several choices: destroy the planet and retreat or colonize it and build it up again or use a lot of resources to get the drop ships in there to take over a planet. Invading a planet should take considerably longer time than simply destoying a planet. And there should be researchable defences only directed against those drop ships. The drop ships are only build for speed and armor, they don't have any weaponry. They are single use crafts and need to be rebuild after usage. With a research upgrade, those drop ships might also work as boarding ships to tae over enemy infrastructure and capitalships...