Making the defense platforms automatically repair or having a "repair all" for some alloy cost would be a really solid improvement. If they need to be an alloy sink because they need to compete for resources with ships, then that's fine.
In their current state, defense platforms are too expensive and too tedious to use.
I have suggested this very idea before, but now that I think of it, this isn't worth it. It'll still be pointless to build and rebuild platforms, purely because of them being alloy sink. Ships are alloy sinks in wars as it is, and ships are the most active source in wars. They get destroyed and have to be replenished etc.
However, stations and platforms only engage when enemies are present to capture the system. The concept of having semi-permanent defense platforms that gets repaired along with the star base, and does not become completely destroyed, would also show the significance of such a heavily defended system.
Defense platforms are infrastructure and are "part" of the star base's defense network. Right now they feel like temporary measures, which are not worth the cost. If you take an enemy's fortification, you can technically repair and turn it against them. That's what I was suggesting basically, to sum it up.
As a trade off, though, there's still the upkeep for them.
As a side note, it seems a bit ridiculous to have separate stationary platforms around the star base, which is located at the center of the system, where the local star is. It'd be logically more ideal to strengthen and enlarge the star base itself rather than having to invest heavy loads of alloys on temporary measures like the vulnerable and separated platforms. Even pillboxes away from FOBs are temporary, but they are cheap to maintain BECAUSE they are temporary way to secure locations. However, the defense platforms are not. They aren't worth it.
It's better to have more solidified structural integrity with additional weapons than divided / weaker structures that cannot maneuver..
If that semi-permanency bothers people, then at least allow the enemy that captured the station dismantle its fortifications.
Finally, I'd just like to add that I respect all of your opinions.