On our different takes above, alm - to me, it's pretty obvious we're both correct. We just adopt points of view from two different paradigms, and have not truly shifted our universe/domain of discourse to really meet.
Let me flesh that out a little more. Typically what this paradigmatic shift means in physics is, if you apply a limit condition to a certain paradigm, it needs to blend seamlessly into the other paradigm for both to be correct (see Special Relativity vs. Newton's Mechanics).
In our case, with your "highway speed" point, alm, you assume all of these decades-old instruments still have relatively large (or: "significant") drifts. And maybe they do - I don't have many years experience in this field - but I have not seen them drift in months' long, daily observations accruing hundreds of hours (as I said). I may still be wrong (maybe my methodology is faulty), but if they still significantly drift, I should either observe it, or they all drift exactly the same (which I think is impossible, as they don't share the same references, design is completely different, even their age is likely different, etc.).
So if you take your assumption of drifting and instead assume it's insignificant, my point on accuracy and reliability of my bench volt becomes valid.
To bring a metrology point in this conversation, let's look at what Solartron-Schumberger says relative to their 7081 8.5 digit meter (see attachment). I'd love to hear others' experience with drifting references (particularly decades-old), but SS states past nine years the drift is "insignificant." This is on a chart including sub-ppm quantifications, and for an instrument qualified to measure sub-ppm quantities.