there just can't be 'another' t-40 for so many reasons.
the t-40 in an of itself was so insanely unique. it was sold at a low price, it was made in USA, it had a mind of its own. As time went on, many folks realized their potential and the price has gone up year over year for about 15 years now - the last 8 or so the steepest incline.
you have to remember, during the era of the T-40, there were not many budget instruments around. The ones that existed really were very bad. This is a key point - now more than ever, the market is flooded with budget options - many of which are really, really nice.
I have a harley benton bass (yes, i've swapped the pickups and had the frets dressed) that out-performs just about any jazz bass i've ever played. the bass was $350. but, you can't expect harley benton to become a valuable instrument in generations - because it's one of... dozens? a hundred? hundreds? of cool budget options. I have a friend who's sold his expensive strat because he prefers his $300ish donner strat. same story.
the perfect storm of quality, unique character/features, budget option, not many other quality budget options... it just can't happen again?
there certainly are things laying around that will be more valuable in years. but i doubt it's what any of us can guess.
i'm starting to see more appreciation for the peavey cirrus line - but those are already well over 20 year old basses, so they're not a 'bass from today' that will be valuable tomorrow either.