I think there are three things that tumble in value very quickly:
1) display devices (monitors, projectors, etc.), particularly in the face of new emerging standards like 4K, Rec2020, and so on
2) hardware players (DVD, Blu-ray, etc.), particularly as physical media continues to fade out from the marketplace
3) surround components (receivers, decoders, etc.), as standards change and new things get added, like Atmos.
I have a couple of HD projectors lying around here, both so old they had no HDMI connection. Each cost well over $10,000 in its day. I'd be very lucky if somebody would come by and give me $100 just to take them away.
And I also have at least a couple of $3000+ DVD players lying around. They're very well-made, they still work, but who wants to look at standard-def today?
My old maxim is: "everything in consumer electronics eventually winds up selling for $50 on eBay." I don't care what it is. What's particularly sad is when you have (say) a $5000 state-of-the-art surround preamp from 2005. No 7.1, very little HDMI support, no Atmos, no Auro... that's gonna be very tough to get rid of. It's kinda like buying a $250,000 Ferrari and knowing it goes down $25,000 in price the moment you roll off the lot.