Maybe - but not many worlds are that 'inactive' - some of those I'm on can't must 50 in the overall roster anymore, let alone in one alliance.
They obviously way up the cost of maintaining and managing old worlds (in general) versus their loss of income and at present the income wins.
They're not pure gravy because there is a cost of developing patches that work on so many different variations of worlds and testing them for each type as fully as they can. We know from bitter experience that they don't manage to always get patches right, even for new worlds.