If you get good units, they will probably last "forever".
Nice setup in ur pictures. The only tip I got would be to not strip/tape the power extentioncords together like you have in your picture.
Getting 30 S7's in there, and putting them together like that would probably generate some heat!
Happy new year!

nothing last forever, every HW is subjected to thermal stress, and they have a precise lifespan, this is the case for gpu too, even if well cooled after some years they will break