The 80/90s mountain bikes were the simplest designs that used decent quality components. Most of the components are still able to be replaced by parts used on current low end bikes. Why do low cost bikes still use them? Because it is much easier (and cheaper) to mix simple generic parts.
When rural touring, it is more likely to find a store that stocks generic low end parts than it is to find higher end parts that are system/model specific.
I tour on a modified 80's mountain bike.
Does it have best performance and is it the nicest to ride? Probably not (but I like it).
Can I find a derailleurs, wheels, tires, stem, etc, for it anywhere? Absolutely.
I can live with that trade off.
I would rather finish my tour with a crappy part than not finish.