Are "vintage" DAC's worthwhile, or is this a tech that does not age well

Doesn't matter what side you're on it has no affect on the DAC. It either measures beyond human audibly so you are hearing the other components in your system or it doesn't. DACs have been a solved problem for a while now of course this doesn't mean some companies create filters or add tubes to create what some term pleasant distortion. 

Just ask yourself the question whether you would buy a 15 year old PC: there is your answer....

....most likely no asynchronous and certainly no high speed USB, no precision clocking, no DSD or high res decoding. All of this matters because you ideally want the DAC to slave the source to minimise jitter.

one doesn’t need golden ears to hear that different dacs present the music differently, just properly working ones... 😂.... but folks will believe what they want to believe and some can find reason to not trust their own ears

that been said, for most in this pursuit, a modern dac makes much more sense, given the compatibility to all sorts of input formats (most specifically usb fed from computers serving as the streamer) and bit rates and resolutions of various audio files folks may use or buy in the present day

that doesn’t mean that older dacs are poor sounding, quite the contrary ... that is a different question -- as with anything in this hobby, one just needs to know what any item does, does well and does not do...

notions of power supply isolation and jitter management are not recent discoveries, recently addressed through recent technology -- for example, even peter madnick’s very affordable audio alchemy gear from the 1990’s was properly addressing jitter, clean power in digital and proper transmission of digital datastreams

In my opinion the analogy of comparing a 15 year old PC to an older DAC is not a good example. If the priority is good sound quality then numerous vintage DACs are still desirable. This has been confirmed by many responses in this thread.

If the priority is functionality and connectivity then they do have some limations relative to current production DACs. For me,  Redbook sound quality is the more important criteria. Depends on what you're seeking. 
Charles