Conversely, since many of today's highly-regarded makers are somewhat famous (see Per Brandin) for longer, lighter, hollow-built rods, one can surmise that shorter rods from these makers will eventually command a premium, as well, because fewer of them will come to market.
I liken the short-rod phenomenon akin to a "buy & hold" strategy in the Market. The Market always goes up long term, and if you're willing to hold on to what has always worked in the past, you're likely to be rewarded. For investors, short rods are the "buy & hold" of the bamboo world, and are what I would search out if I bought rods purely for investment. However, we shouldn't lose sight of the fact up & coming fly fishers have cut their teeth almost exclusively on 9' graphite rods. Perhaps there will be a future market for longer bamboo rods.....
I personally like fishing with longer, light bamboo rods. And I find them superior to lifeless graphite for aesthetic pleasure...graphite will simply never feel the way bamboo does when fighting a fish or when you need to land that small dry fly ever so delicately in the feeding lane of that Stanford-educated spring creek riser. However, my idea of a long & light rod is 8'6"...maybe 8'9" max.

Wonders never
cease.