I get what your writing though, measure a single instrument throughout it's use, decade after decade. It is more feasible than having pairs, and one locked away, while another is played and measuring / comparing them years later. With one instrument though, it can be hard to keep track of what change is causing what. Instruments being used, will be modified and adjusted according to best needs of the current performer.
Yeah - but a lot of people talk about fiddles opening up in the first months/years of heavy playing. My wife is a luthier, and when I asked her this same question, she said she suspected the sound post and everything "settled in" rather rapidly. Sorta like a new car's squeaks settling in. I dunno - that sounds like it COULD make sense to me. But again, I think there are countless references around here of people saying their instruments changed/improved in less time than a decade - which suggests a time period that is very testable.
And people talk about the wood structure changing. Again, that SOUNDS like something that COULD make sense to me, but if it were so, I'd expect it to be testable. Presumably, string instruments are made from well seasoned wood. My bandmate's new cello was made from 150 yr-old wood. I'm not sure how much that wood is gonna be changing anytime soon!
I sorta suspect that with the buying/selling/assessing of instruments, many people DESIRE a "fudge factor" - describing things in non-verifiable terms that can justify a higher "value" for an instrument. I agree, there is some allure/romance associated with an older instrument. I've got no objection to someone saying that sorta thing. But when someone says that time resulted in some specific sonic improvement, that impresses me as something that could be independently confirmed.
Couple more points - I obviously have a tin ear compared to many of you around here. Hell, I'm a bluegrass thumper in the pro-dot faction!

Hell - I had to think a bit before I figured out where the B WAS on my E!

And my current bass is a 10 yr old plywood Englehardt. So I have no personal thought as to whether that hunk of plywood sounds any better than the day it was made. If it does sound any better, I'd suspect a large part of that was due to my improved ability, and my wife tweaking the setup...
Kinda relevant, tho - I've taken my wife to look at a bunch of old plywoods, and her opinion is that old DOES NOT necessarily mean better. When she looks at a 20-70 yr old Kay, she sees a bunch of real or potential problems. To a large extent (not all) luthiery involves creating a solid resonating box. And modern makers, using modern tools, are certainly capable of crafting solid boxes.
The old basses definitely call to me. But my wife says, if SHE is going to be maintaining them, she'd prefer that I buy NEWER!
I'm getting a new Upton hybrid sometime this spring/summer. I wonder if it would be at all practical to try to do some amateurish tests of my own? Just making recordings at different times, and seeing if I can run them through some available software...
One final question - are there any makers who have made similar models for long periods, without significant changes in their tooling, sourcing of materials, quality standards...? Say Shen - 20 years from now. Will a 2040 Shen Willow differ from a 2000 Shen Willow (assuming consistent setup)?
Thanks all for engaging in this discussion in the spirit intended.