Building PDCs was fun. With the new orbital weapons systems this could be even better. And having ground based fighters and Missile systems would make my day. Why were they removed? Any thoughts? Better ai diplomacy with more options would also be great.
Because they were just stationary ships you had to build in place. That's literally how they were implemented. If memory serves, they were also easily detected by ship sensors, as a ship would be. Thus, orbital defense bases (the only way to implement missile launchers as static defense for a planet,) are functionally identical to a PDC. They don't get four free layers of armor (big woop) but don't need to be painstakingly built
in situ either which more than makes up for it.
The biggest advantage of C# ground forces is that they can actually hide on their planets - they're hard to detect from a distance; a ship has to close in quite a bit to pick them up on Res 1 sensors, and even then STO forces can lurk and let the enemy get closer before unloading. (Some STOs can even stay silent to present a persistent threat to achieve aerospace denial, much as modern mobile SAM systems do IRL.) For a ground-based missile launcher to be meaningfully different than an orbital base, they would need this same ability to hide.
There is no reason this couldn't be done; but we would have to think through the melding of the ground forces and space combat rules should go to make all those components work together, much as was done for STO batteries. I can understand why Steve would see this as a low priority feature, even if he had inclination to code it.