Even if such a ward is created, it's too easily broken by any non-undead. And if set against creature types of possible living servants, it actually traps the townsfolk and PCs as much as it keeps the Lich Queen's forces out. There is no "friendlies can pass" or even "I can will the circle to end" clause in the ritual. You could trap yourself forever in your own circle if you're stupid enough while casting it.Enforcing the rules of the game is not being a dick (even if your players complain).
You can't cover an entire town with a single casting of Undead Ward, period. Sure, take a month and blanket it with 30 castings... costing more like 1000 gp in components. This is available to level 6 characters but it's a serious chunk of change.
Well, a case could be made about the rituals that a wizard and some other classes automatically get while leveling.In any case, the main difference here with 4e is that PCs don't automatically learn rituals (except for a few specific exceptions). They are by default unavailable. So the "magic arms race" is entirely at the DM's discretion. If the wizard learned Undead Ward, it's because the DM made it available. If the DM is making Undead Ward available in the same campaign world as a lich queen with undead armies, it's the DM's responsibility to sort things out in a way that makes sense.