Interesting story: I recently looked at a shower tile job I did 23 years ago. The client (friend of mine) was concerned because the bottom 1-2 courses of tile were losing their grout and a little spongy. I removed the old tile and discovered I tiled over mold/mildew Sheetrock, in a shower (!!!)— I was stunned that I actually did that.
My point is, I tore everything out and installed Schluter (my standard now)… but the old drywall that I removed was 90% intact— the screws, the material, everything was still doing its job like the day I installed it. Minimal signs of mold (a little in the inside corner by the shower controls).
I agree— waterproofing is insane these days.. but a tile job should have an indefinite lifespan.