Downside to R2R Ladder DACs?

George, this is very true - two power supply caps in parallel have smaller ESR, but they also have smaller inductance, often horrible for very large caps.  In addition few smaller caps instead of one large cap allows for better packaging (more efficient use of space).

Point made by hrabieh is very good - DAC is not only D/A converter but far more.  There are many DACs based on the same D/A chip that sound differently and have different reliability.  

George, this is very true - two power supply caps in parallel have smaller ESR,

And it’s great we can access cheap ones from our little Chinese buddies who make them complete ready to go at what just our parts cost would be. Many more variation on this theme in ebay voltage and capacitance.
https://www.ebay.com.au/itm/DIY-kit-62cps-Nichicon-330uf-100V-Array-Power-supply-kit-for-Power-amp-D...

Cheers George

R2R DACs are dependent on the accuracy of the resistors. Therefore if a particular design has a long term problem with one or more resistor values changing (absolutely or relative to the other resistors), then the DAC will be degraded. I suspect that unless the degradation is gross, most people would not hear the difference with normal music at normal listening levels. So saying "my 20 year old R2R DAC sounds great" is probably not a valid test! However, the degradation would be measurable and that would be a valid test. If this was a common, significant problem, I would expect there to be a lot of published engineering articles demonstrating it, but I've not stumbled on anything yet.

cheeg your dealer doesnot sell any high quality R2R dacs 
that’s why all the misinformation a Lampizator, msg, Aqua ,
Denafrips .and and a bunch of otherbrand name  dacs made Today Are excellent . Waay too much misinformation .
it all comes down to the engineering and implementation 
of the design,your dealer is Totally bias ,I learned 40 years ago 
never put all your eggs in one basket ,there are a Many dealers online !!

Plenty of electronic components will drift over time.  You probably won't hear it s it is gradual.  In direct comparison if you had a time capsule and kept a new one 20 years later to compare, you may be able to hear the difference.  Not all R2R boards are the same so a general statement is that it won't be a factor.  Is your salesman saying that a chip set won't also degrade or drift over time?  Well at least you will be enjoying your music more until that time comes!

Happy Listening.