I think that there's an existence tax for any software; if you don't spend a certain amount it just stops working. On top of that Skype is out in the wider world and so faces maintenance costs that a lot of enterprise software doesn't; ux updates, OS updates, security, interfaces to network kit and so on. If you have a nice pipe of revenue then this is all affordable and extra money can be sunk into making it good... but I suspect that Skype just doesn't have the revenue stream required to support that.