It'd be way too expensive to have literal duplicate animatronics sitting around -- some of them cost a million dollars apiece or more. (For reference, the original hydraulic Imhotep I cost reportedly cost roughly 4 million, with Imhotep II and Reggie making up the other 3 million of the AA budget.) However, in the "good old" days Disney would keep multiple parts for pretty much everything ready-to-go, so when something broke, it'd take a night or two to repair. Now they don't order anything until it's already broken, which means it can take weeks to fix certain elements.
Universal used to have that point system where an attraction couldn't accumulate 10 or more "points" in broken elements, otherwise the ride went down til they could get it under 10. That process seems to only really apply if it's convenient these days, I'm afraid.