Just my humble opinion, but you might need to consider finding another mechanic.
He seems to have a pretty ill-informed and casual attitude about destruction of a steering rack.
If you’re going to get rid of the car soon then just keep the fluid topped off. No sense is going into it at this point.
The rack boot should have been replaced when torn; pure and simple. Once dirt gets inside those boots it doesn’t take long for the rack itself to get scratched. Once it’s scratched there is no fix.
Fluid with sealer may, or may not help. That’s generally for a leak due to a faulty seal but will not cure a scratched rack or a torn seal due to a scratch.
“it just keeps dirt from getting in there so it’s not a big issue”. Jeez.
By the way, changing a rack boot is normally not a 500 dollar job and changing a steering rack is not normally a 1500 dollar job.
Is there something in this customer/mechanic relationship that has not been said or does he just not like you and wants to brush you off?