The length of "forever" is inversely proportional to how old you are. A great DAC bought now will still sound great 20 years from now, assuming the electronics and mechanical devices within it don’t age and change value or lose function over time.
It is best to look at our 2-channel stereo set ups as SYSTEMS. If I have a $10K stereo overall, the DAC I choose to get likely will be no more than whatever the "best" one is around $1000 to $2000.
It would be foolish of me to buy a $6000 DAC when my system (and my older ears) won’t be able to appreciably hear a significant difference.
It all depends on what you are chasing and how often you are mentally and financially willing to try catching it.
Technology will continue to improve to a certain extent. Today’s high-end DACs certainly are amazing devices, but that level of performance will eventually trickle down to cheaper ones and the difference between "good enough" and "great" will continue to narrow.
Think of HDTVs. Just 10 years ago OLED and other technologies cost a premium, but today QLED has caught up and even surpassed OLED, and prices have dropped tremendously. The march forward of technology is a great thing.
Consider that the current darlings of DACs, the R2R types are mostly made in communist China because manual labor is cheap there (like many of the popular tube amps requiring point to point wiring are). But imagine a company investing in automation to create high end DACs and do so in mass quantities. That might happen one day and then even the great unwashed out there who don’t even care about sound quality will have no way of avoiding it and actually hearing it.
It will just be the technical norm at that point.
As others have noted, just buy a good DAC, price commensurate to your system or thoughts on how far you plan to upgrade say in the next 5 to 10 years, and simply enjoy the music.
Aside: Regarding "forever" I note that many of the Denafrips and other DACs use electrolytic capacitors in them. Should we be concerned about them drying out and changing value over time, like the ones in old speaker crossovers? IDK.
Is there such a thing as a FOREVER DAC/streamer?
I know I might be talking about rehab in a back alley, but here it goes.
I've been looking to upgrade my Hegel H390's internal DAC. But I would only do it to knock it out of the park; as the Hegel's internal DAC does an admirable job. So the question is, is there such a thing as an end game-forever DAC. At the end of the day, DAC is a digital product. Digital technology evolve quicker than analog technology.
Will that Lumin X1, Bartok, Dave, etc be obsolete in 3 years? Thus making this a moot conversation...