Shop is merely trying to cover all bases. With AWD you’d risk damage to (expensive)parts of the drivetrain by adding a new tire when the old ones are most of the way through their lifespan. Depending on how much tread is left on the old ones, you could simply buy a new tire of the same kind you have now(making sure all 4 tires are the same) and have it shaved down to match the other tires.