Re: they last pretty much forever
>BOLTS not screws
Cage nuts are a comparatively recent innovation. There are still racks around that have round holes for screws. The fun and games is determining which Standard was used and sourcing the relevant bolts/machine screws.
Mind you most racks contain a good mix of ISO threaded stuff and US imperial: A set of racks I've recently had installed are all ISO except for the UPS rack rails that use UNC (machine) screws...
>a 21U rack was literally put together with self tapping screws.
Yes, there are cheap racks... and there are racks that use some surprising easy to loose but hard to source special parts...