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

The current generation of DACs are all measurably better than their predecessors, but you may not feel that is important.
There is better understanding of clock architectures including phase behaviours. There is also better understanding of how noise is handled.
For Red Book discs and files (44100Hz), how the "brick-wall" filters are implemented should have audible impacts.

DAC chips age well, in fact many of the best DACs made today employ old chips. My personal favorite is the CAD 1543 MK ii which uses 16 of the old Philips 1543 chips. 
Advances in understanding of power supply isolation, usb integration, and EMI/RFI, mean that new DACs using the old chips is often the sweet spot. 

I have also heard dacs sound different from each other. I even had an old dac upgraded and it became much better.

Regarding the question I would guess that something like a ML dac that was very advanced for its time still sounds good. Maybe not as good as the latest equally advanced and expensive dacs. If you can get it for a good price it may be worth it.

On the other hands there are now dacs for $1k - $2k that are said to sound great. You would probably have to compare the old and the new next to each other to hear which is best and which one you like the best.

Here is an example of a somewhat affordable dac:
https://www.stereophile.com/content/okto-dac8-stereo-da-processor

If you for some reason don't trust Stereophile it also got a great review at ASR
https://www.audiosciencereview.com/forum/index.php?threads/okto-dac8-stereo-dac-review.14705/

Apart from ageing parts i do not see any reason for not sounding excellent in redbook. I bet a SFD2 can show a clean pair of heels and then some to modern dacs. And yes there is a big difference between a 5k to a 0,5k value dac.

G

Firmware Updates to CD/SACD/Blu-Ray players.

I typically update my camera's firmware, but for audio use of these?

I just bought a 2009 Oppo BDP-83SE (special edition, improved audio).

https://hometheaterreview.com/oppo-digital-bdp-83-universal-player-reviewed-1/

It has Firmware Updates, I suspect only for minor video issues. 

If all the initial reviews said very good audio, never found an audio 'problem' mentioned, why do/risk it?

Any recommendations?

thanks, Elliott