And who told you this?
What they told you is wrong, but there are cases where the unit will need replacing when it leaks down. EG; if the coils or lines are so badly corroded, rusted out compress dome, salt water corrosion.
Any A/C tec can vacate (get rid of the water inside) the system with a simple vacuum pump, recharge and check for leaks. That's A/C repair 101.
There is nothing inside an A/C system to gum up. There's Freon and a small amout of oil to lubricate the pump that's it.