The problem with stockpiling parts is they sometimes don't do well over time. Metal parts if kept in a controlled environment do well enough but in bad environments, well we all know what happens to metal combined with moisture. Anytime you get a metal part that has rust or a white coating then the structure has been compromised.
Things like rubber and plastic can be a waste of money to stockpile. Take ramps for instances (something I know quite a bit about), if you buy NOS ramps they can be up to 25 years old. You don't know if they were stored in a controlled environment or some non controlled environment warehouse.
The plastic that is used today on new ramps will do well for about 10-15 years in a good environment. After 10-15 years the plastic starts to get brittle even in the best environments. It can still last 20 years or more if its really been taken care of and cleaned with the right type cleaners. In a non controlled warehouse space or worse an outdoor shed or container the plastic can go start to go bad in less than 1 year. That's one of the reasons we store all our stock in controlled space.
NOS ramps are really a risk as most are beyond the shelf life, especially if you don't know the history where they have been stored for the past 10-25 years or even the last year.
Rubber parts are the same risk, rubber rings that have been on a shelf for 10 years are worthless. They will break apart when being used on a machine and the small bits can clog up the mechs under the playfield. It’s always best to buy rubber rings fresh and new.
Coils are generally good, not a lot to go wrong there so stocking up for the long haul isn’t an issue.
Playfield plastic is a butyrate and starts to smell like cat piss when it starts to break down. Again the shelf life is finite and depends on the environment.
Backglass…nothing needs to be said here…environment, environment, environment…
Translights do seem to hold up a little better than ramps and PF plastics as they are made with mylar material but some inks can break down over time or in bad environments.
Electronics…usually do well expect for large capacitors. They are a little like batteries so the shelf life is limited.
