I havent really experienced the effects of blockades yet, but i agree with DownTheDrain, making it that local defenses will deal with the blockades when they equal or overmatch them would make sense. It doesnt make much sense for local defenses to throw themselves at a blockade they cant destroy, but it makes even less sense for local defenses to not attack a blockade they can destroy.
And as vicegrip kind of mentioned, factions already send out ineffectual missions all the time, especially at the start of the game.
My view of seeing the core worlds churn with raids, invasions, and blockades is that it gives a feeling of active combat and things going on, even if in 90% of the cases they fail. Things are happening.
If invasions were weighted as such that factions only dedicated resources to actions that are guaranteed to succeed, the feeling would be entirely different. And unless they were slowed down to the point where only one or two of these invasions happened a cycle the core worlds would feel too volatile.
The factions throwing away men and ships at actions that just fail is a bit silly, but i dont mind it. It keeps the sector feeling active.
A decent idea for a new feature would actually be able to select from a few different activity presets during sector creation. A good option for one of these settings, perhaps it could just be the one alternate, would be 'total war' or something similar where factions only send fleets that are 50/50 or higher to actually succeed, but will do so more infrequently.