Restating your point. Some of it is a time issue. My US L2500 was $1050 brand new. I got a decent deal on it back in June of 2002 (shipping from Carlsbad to Indianapolis only cost me $35 back then too). Confident I could get at least that used.
The other drivers are going to be scarcity and desirability. The G&L L5000 has always been somewhat rare for G&L (roughly 400 produced). However, it is not super desirable because string spacing is tight. I have seen prices on mint examples of these climb from the $800-$1000 range to the $1200-$1400 range in the last 10 to 15 years. I love mine and would like to see these catch on with players, but I do not expect it to happen.
I think it will be interesting to see what prices do on some of the more recent rare builds. Basses like the Matador, Matador 5, and CS Fallout Bass with 2 MFDs. Historically the exceptionally rare G&Ls bring a decent amount due to rarity. There are enough G&L fans to drive the price up on an instrument they only made a handful of (handful meaning something like 25 or fewer). The ASAT B5 has always been a good example here. They sell quick and the recent ones have sold for over $3000*.
*$3000 is good for G&L, but still not close to vintage Fender territory, particularly on a Fender that is as rare as the ASAT B5.
Actually, it sold for $3500. That included a second neck, fretted, that I picked up cheap from the G&L garage sale site. It was an incredible semi-hollow but far from how it left the factory. A prior owner had swapped out all the brass/gold hardware to another B5 and the fretless neck it came to me with was a later non-B5 neck. Still, should have kept it but I've said that about so many basses, the majority US G&L's, all purchased used.
"My US L2500 was $1050 brand new. in 2002...Confident I could get at least that used."
That's $1800 in today's dollars. So you'll lose $750. Simple economic math. Keep it, that's from a good era and players want 5vers. And apparently, it's your only MFD-equipped bass.
Many have no idea of inflation's actual cost nor what "in today's dollars" means. That $300 Fender purchased 25 yrs ago is $565 in todays dollars, which is the amount it'd have to sell for just to stay even. Selling for $565, a seller didn't make a dime. A pre-CBS Fender bought used for $200 in 1975 and sold 45 years later for $6,000, is a different story.
A run of the mill (Fullerton) G&L bass bought new at this moment (and with no warranty, NONE of the ones listed are actually "new") is going to take a long, long time, if ever, to not lose $ on. 20% off a new with warranty one was fairly standard up until G&L's SHTF.
Own several with that two-way truss rod? Buy some stock in Musikraft necks. Investment? Buy a house and/or rental property. Or that Shelby Mustang.
Love the G&L's you own? Play the

out of them and be happy.
Then again, don't listen to me, 6 months ago I sent this to Hawaii. Before you ask, no, it didn't neck dive any more than any other ASAT bass and semi-hollow's dive just a little less.