OP: “Given that the age of the vinyl stamper impacts the SQ of a given copy of an LP significantly, wouldn’t it be true that if you stick with a single digital source, e.g., Qobuz…. ”
That is just one variable. Old albums can be remastered a number of times, or not mastered but produced from a secondary transfer as master. It is extremely complicated as to what you get.
On the other hand, in general if you double the cost of your system all the recording sound MUCH better making typical recording variance less important. So, it is a matter of proportional differences.
Also, when your only source of music had to be purchased… at a high cost per unit (I remember only being able to purchase one album every few months, or none) you tend to be really careful what you buy. Also, exposure only came from radio and friends. This tends to force drilling to find the very best music and recording and listen to it over and over. But in the digital world, you can sample tons of stuff… and follow interests. Don’t like the album you found, for whatever reason… move on.
Also, the catalogues are only going to get more robust. I am sure different mastering are in the future of streaming and more and more obscure stuff. So, to me, this still points to digital as the future. Although I have enjoyed the hundreds of spectacular vinyl recodings I have, some original and many audiophile.