Was away for quite a while so it seems I missed my chance to say my piece. I don't think they should be removed. However, I think their mechanics need an adjustment and an expansion. A PDC is really meant to fulfill some key functions that are a bit odd to simulate with other options (units/titans).
At its core, a PDC is a fortified location to counterattack/defend an area against enemy starships. While it is entierly feasible for units and titans to also do this (The dragoons of the Ember Wars comes to mind with their shoulder mounted rail-cannons that can swat lighter starships out of the sky), they key words are fortified location. Units and Titans are more meant to be a mobile force to eliminate enemy mobile forces and strategic positions. PDCs are like that to protect key facilities, like planetary data centers, emergency power plants, and planetary shield generators (more on this in a minute).
Reworking PDCs so they better fit their role of protecting key facilities/areas would be to add something more that only PDCs could use. One of those things could be a planetary shield. Far larger and more powerful than regular shields, these shields are meant to hold back a lot of firepower. Maybe weighing in around 5,000 tons, 500 times the weight of standard shields yet gives around 750 to 1000 times the strength. The shield also can take damage from enemy Titans' artillery rolls. The shields wouldn't block enemy troop combat however, so there is a stronger point to invasion over outright planetary bombardment. Possibly to simulate the fact its ground based and can only "cover" a certain "area" around it, there could be a coverage percentage based on the number of different installations and the habitable area of the planet (with small enough bodies allowing a single installation to cover its entirety so small moons/asteroids aren't requiring too much).
Another thing would be anti-titan artillery. I actually have no idea how this would be figured, but it may be its own technology that does its damage based on ground combat strength tech. This weapon could also be used against enemy troops on the planet that aren't already in combat with that PDC or inside another one.