After 3 850/70s, I have never replaced a strut mount even approaching 200K miles. Spring seats are made out of a rubber like material and do disolve with age and climate.

Your mechanic is going to love you!! The strut has to come out of the car, the spring compressed, the spring seat replaced, spring expanded, and strut remounted. Same procedure as replacing the strut. It takes me 90 minutes each side to do this, your mechanic has a lift and air tools and can do it in less than 1 hour each side. The longest part of the job is compressing/decompressing the spring.

I bought a spring compressor a few years ago and it has paid for its $55 cost. Of course, to do the strut removal, you also need a floor jack and jack stands.