The problem with Xenoscepter's suggestion on how to improve existing GSF is that it introduces several new special rules, which is a thing that Steve has wanted to get away from. In fact, reading your post makes the new GSF sounds very much like the old PDC, in that they are a ship but, on a planet, and with a bunch of exceptions affecting them.
The scale issue is certainly real and for that purpose, as well as the ability to better portray current, past, and near-future conventional armies, I'd love to see GSF become a 'proper' ground unit, especially if auto-assignment is introduced.
For all the reasons other people have already explained, keeping the space and the ground parts separate is probably for the best.