This is a harmonica I customized several years ago that was sent in for service.
It is evident that it is well played. Before I did anything to it, I assessed it. The draw chord was still in perfect tuning.
Why is that?
Custom harmonicas are much more efficient at converting air flow into sound than stock instruments. They promote good technique and breath control. As such, they last much longer than stock instruments. I get asked if reed work weakens the reed.
With proper reed shaping technique and intelligent methodology where the customizer does not work against themself, no, reed work does not weaken the reeds.
That being said, all things have their limits. When the time comes, a good custom harmonica can be serviced and the reeds restored to like-new condition. You do not have to throw away a custom harmonica nor does it cost very much to have it serviced.
https://harp.andrewzajac.ca/Custom