So, is that line of thought just a relic of mountain biking history? Or am I missing something?

I recently bought a 2005 giant Iguana in your price range. It has a 100 mm travel marzochi exr front shock
that used to come on more expensive bikes. It's got an alivio front deraillure which is a a couple of steps
better than the acera that comes on the trek and I'm not to happy with that. but overall it is a better speced bike and there are others out there. I'd suggest visiting more shops and comparing drive train
components on each bike. You should be able to get a 27 speed with upgraded deraillures for under 6 bills.
then after you ride it for a while you'll find out what you missed,like most of us.