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

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.

Just two thoughts:

1. It depends upon your age.

2. IMO digital audio is currently more mature than it is being given credit for.  It is being credibly compared with analog and with the sound of real music.  The really good stuff at various price points is there for you to discover.