10-15 year old hi end DACs are still great.

audiophiles weren’t deaf 15 20 25 years ago... what sounded pure then will sound pure today

there is newer technology available today but cost cutting/value engineering has also been optimized -- so newer stuff is a dual edged sword -- no assurance the sonic performance will be better

I think as long as the older equipment's components are still good, the sound quality of much of them are up to or equal to todays products.  I went to the Axpona show this year, after being out of the hobby for over a decade.  Heard some fantastic systems.  But to be honest, they really didn't sound any better than the fantastic systems from 15 years ago.

Also listened to some systems that were using class A/B amps made by companies that used to make Class A amps - often todays A/B versions did not sound as good to me as the former Class A amps.  I am not sure where all the Class A amps have gone and why they have  become so rare (and of course, so expensive). 

Surely many will suggest a selective memory, but I have to be honest, I've heard several new DACs that are highly regarded that are no better than the level of performance of DACs that I owned over a decade ago.

I would have thought the easiest improvements would have been in the field of DACs and Digital, and there certainly has been some significant improvements in those areas - and yes, digital has gotten better to some degree.  But I have not found that just upsampling always results in better sound quality and enjoyment.  That said, there are more "mid priced" and "entry level" digital products that have improved over the decade ago entry and mid level products.  It's easier to get acceptable sound, but not so easy to get superlative sound performance.

My experience is that older DACs often have excellent power supplies and analog output stages. Those seem to be areas were designers skimp these days, especially with just using simple opamp output stages that measure great but don't sound all that amazing in a lot of cases.

On the other hand, older DACs will often be limited on their USB capabilities, if they even have one. Which can be overcome by using a good DDC to convert to AES or whatever you prefer.

I still have a Luxman DA-06 from over 10 years ago. Original price maybe $6k, now sells for much much less on the used market. I've heard it beat many new DACs, especially when it comes to midrange body, layering, and richness. Same goes for my ModWright Elyse DAC which replaced the Luxman. 

My experience is the contrary. Not long ago I offloaded an Electrocompaniet ECD-2 ($3000 and SOTA measurements in 2013) because the DAC in my $320 WiiM Ultra sounds better. Concerning DACs, price has very little, or often zero, correlation to performance these days. 
 

With a few exceptions, I’d think really good sounding stereo hasn’t Earth shatteringly changed in the last couple of decades. Delivery Systems changed, the way we deal with bits changed, but a really good rig from 1999 still sounds like a really good stereo. A Jadis Defy 7, a dCS Elgar Plus...etc. I’d imagine it all still sounds pretty good. Why wouldn’t it?

I mean, a lot of preamps, for instance,  came and went along with a lot of technological marketing bluster but a Krell KRC HR (I don't even remember what year it came out) still sounds like one of the very best preamps I've ever heard.