Structures cost a substantial amount of money.  It's quite common for a planet to have more money expended in its structures than in the planet itself.  They aren't that hard to destroy, so frankly I don't think they should just disappear (even slowly) if the planet is lost.  I know people are in love with fighters, but this is what bombers are for.  Ever seen what half a dozen bombers do to structures?  Great if only for quickly destroying repair bays.

Well, you'd still get the planet vision with ferang's suggestion, Orodum.  It just wouldn't be indefinite.  And, that's all you have to sacrifice to be able to get rid of allies' structures when you take over a planet they lost on their own.  If you just wait, their structures will just go poof.  Minus the idea of "surrogate parenting" anyway.  I'd say the only way the "surrogacy" trait should apply is when the structures are also taken over by your forces.  So, not only would you need a parent asset in the grav well, but you'd have to claim the remaining ships and structures also...otherwise they continue to degrade.  If you got no economic benefit for maintaining these items, that would simply be parasitism.

In which case, Orodum, if you were simply aquired the structures and ships, would you really spend resources to replace things you got for free?

I think that'd solve both issues.

at the very least i would like to see structures and ships that are cut off from a continuous supply line to have a penalty against them.

I'm pretty sure everyone's fine with the game as it is, but the slow dying of abandonded structures would be interesting.

As for the enemy structures in your territory...you're right...but I think the person who mentioned that was referring more to allied structures (which you can't blow up), than enemy ones (which of course you can blow up and therefore aren't a problem).  If allies are already taking up slots in a grav well you can't do anything about them as it is unless you break the alliance.