I could refer to history... No fortress has ever been able to survive for eternity, some day, every fortress has been taken/destroyed, at least I don't remember any which hasn't.
I doubt, it would be that much fun, taking out an AI "super-starbase" requiring huge fleets, and that thing even survives a capital assault. But I would like to see the AI defending their SBs heavily.
Still, the major issues is, what to do on >huge maps, where that many enemy ships are around? As I imagine, huge hulled ships are at least the same size as starbases.
Jesus, get a clue.
Static defenses are overcome because they're STATIC. Tactics and weaponry can be brought to bear against them. Artillery has been used for centuries as a way to crack open a fortress.
HOWEVER - static defenses like the beaches of Normandy did not host a guard unit of thousands and thousands of men only to be overtaken by a few hundred guys with machine guns. It took a lot of effort, manpower, and lives to overcome the beaches of Normandy, where only a few thousand germans had set up bunkers, barbed wire, and tank traps. They had machine guns. The Allies had 100,000 people. And yet the Allies still lost nearly twice as many people as the Germans.
What do you think assaulting a castle took? Siege towers, trebuchets, and a lot of manpower. Some castles lasted for weeks against an assaulting force until they eventually were starved out or the enemy force retreated.
In short, realistic, capable defenses have always been able to stand against a force of equal, sometimes greater strength. It has, historically, taken a lot more to overcome static defenses then it has to set them up.
FINALLY - Starbases are not 'static defense'. If they had an attack range of 2pc or something, that'd be a static defense. However, they can usually just be avoided if they're so scary and intimidating. It's not like it's going to chase you.