So I would say stick with older gear if it does what you want.
Agreed. Yes, for instance, you need a new rig if you want FM, and your current rig doesn't have it. But, often it's not the case.
My 15 year old Midland 73-005A 2 meter handheld has a bunch of channels, but it's so hard to program them I just use the K.I.S.S. principle. It does fine without having features like microtuning IF's. Of course, there's not the pile ups on 2 meters like HF that needs that kind of feature.
And, some older rigs can have things done to improve them, like more sensitive diodes & transistors swapped into the receiver front end to improve s/n ratio, or other filters added. The WARC bands are about the only thing older rigs miss out on, and some of them can be converted to use them as well.
I see things like Collins Linear amps keeping a LOT of their value & demand, considering how old they are.