So if we are going to be remove PDC's, then logistics should be simpler and if we aren't then the PDC's will need a another layer of complexity to them? It would be difficult to keep them as they are now given the new combat mechanics that relies on mobile combat.
The point I was making is to overhaul their systems
alongside the new ground-forces change, not separately. Yes, you could make the new logistics system easier if PDCs were outright removed, yet then you lose some depth in that system. I was suggesting they would make decent/good staging areas much like FOBs would IRL.
Replacing them with a Unit Type with 4 components that is unable to be transported from planet to planet 'might' be a good idea. However, since Logistics units will be drained away, it might not be wise to have them in anyways.
No, having a "Unit" simulate them nor making them immobile are both poor choices. Going to how they are build/designed currently, they are very much like prefab buildings that can be hotdroped at a location then built up quickly by a construction brigade. Under the new mechanics, it could work like this; You droppod in your shock/assault troops to form a beachhead, you then land at said beachhead to drop off your support units and PDC components, and as you are building up your FOB you land the rest of the invasion force which then is supported by your PDC FOBs for when they receive damage and injuries. That FOB could also have artillery and/or anti-vehicle/titan weaponry to secure the area to allow the bulk of your combat forces to then push out and secure the area and invade while you retain a landing site to bring in more troops and supplies.
There is also the problem of how to simulate what happens when a PDC/Base gets surrounded and they are cut of both from the population that maintains them and their outside ordnance supplies.
Same as "mobile" units, they get overrun. The difference between them and PDCs is that the PDCs can pack ordinance, supplies, and food to last months or years while the mobile units will run out pretty quickly. Again, that is why PDCs could be made to fulfill the FOB support route to be able to supply those mobile units with what they need.
Taking one would be basically trying to take another, smaller, planet.
And that is the entire point behind PDCs.