Is there such a thing as a FOREVER DAC/streamer?

At my age I don’t think there is a forever DAC.

However as technological change is rapidly accelerating I would say

not. I would not rely on a DAC to rely on to give me the ongoing audio pleasure I would be happy with. Only my opinion, and it has been discussed on this forum many many times, layer you system by importance. Speakers, then the rest as you feel. My speakers will out last me and they were not cheap. Like cars, electronics will trickle down ( say from F1 or the best Merc or some such) the best will be affordable sooner than speaker technology. Only my opinion. :)

If I was you I’d buy a Gustard R26 on Amazon, and if it’s not a meaningful improvement return it and decide if you want to spend considerably more on something else or just stick with the DAC in the Hegel knowing you’ve got a really good thing.  And, if sometime well down the road there’s a significant improvement in DACs you didn’t dump a truckload on antiquated technology.  You’ll also obviously need a good digital cable, and I’d recommend the Acoustic Zen MC2 (they come up used occasionally, which is how I bought mine).  Hope this helps, and best of luck. 

The simple truth is that many people who have invested in a better dac mostly switch for the sake of trying something new or a slightly different flavor. 

I disagree. Every time I have changed my Dac is to get something better. Not because I'm bored with what I have, a purchasing itch to scratch or a different flavour. I want to experience increasingly better sound.

A contrarian point of view… Yes, you can find your “forever” system if you’re not addicted to continually upgrading gear.  DACs are already delivering in formats that are barely indistinguishable from one another to the human hear, e.g. DSD256 on up.  As long as your system sounds close to live music, what more improvement is there?  Better than live music?  I’m happy with my setup and not planning to invest more unless something breaks.  

Every time I have changed my Dac is to get something better. Not because I'm bored with what I have, a purchasing itch to scratch or a different flavour. I want to experience increasingly better sound.

Exactly.  I've been through a number of DACs and all resulted in "better" sound to me, with "better" of course being subjective.  

Whether something is "forever" or not depends a lot more on your listening tastes and budget than it does on the technology.  DAC technology will continue to improve, but it's likely that those improvements will be small incremental ones and depending on your age and hearing, what you're more likely to hear is the "voicing" of the DAC, not its technical abilities.