Well... "forever" is a long time, but if it's awesome now it'll be awesome for a while... I think many of us can attest to still running on the internal dacs of cd players from ten or even twenty years ago; I know I do, and everybody including me likes the sound of my systems. Is there something "better"...? Eh, sure, I suppose... Will a great sounding dac now still sound great in fifteen years? Yes, unless something goes wrong with it. Might there be new formats, with new "effects" or whatever? Sure, but two-channel stereo will still be there, chugging along and sounding great.
But if you're worried about it (unless you have lots of money sloshing around and your house is all paid off, your cars and all your other debt it paid off and your retirement accounts and kids' educations and so on and so on are all maxed out), then don't spend thousands of dollars on a dac that might give you some small perceived "improvement" over a less expensive one, only to be chomping at the bit to throw another thousands away a few years from now. Find some frugal limit, get a good dac that sounds good with your system and to your aural preferences, and then enjoy the music. There are many many dacs just like many many amps and speakers and whatever else... one can obsess to the point of never being satisfied with anything because there's always something over the rainbow out there; or, one can make some solid selections and relax for a decade or two.
Or perhaps dacs can be "firmware updated" like operating systems... I dunno.
That said, I know what I say is my perspective. Only recently, now that I have some spare cash, have I felt any need or even desire to upgrade anything because I already have two systems that I like a lot. I doubt I'll do much dac shopping; what I have sounds great, and if I do get that Technics SU R 1000 amp with it's own version of dacness, or that Accuphase e-480 with an Accuphase modular dac unit, I think I'll be sitting more than "pretty good." I won't need any external dac; I'd rather play with speakers, if I'm not satisfied with my system's sound. I like good stuff that lasts, and it's been a good formula in stereoland and in-general-land.
