I don't think downfall is a likely, necessary or even sufficient explanationbfor technology plateauing. The question goes, if tech could be better, why isn't it? And the answer can't be "time", because achievements a thousand years in the future of mankind are a thousand years in the past of somebody else, and a million years in the past of somebody else again.
Rather, the answer ought to be distance. Before warp, Earth could plausibly lack transporters, shields, phasers, tractor beams and the like. After warp, interstellar contact should kick in a plateau phase where Earth shares everything with the neighborhood, or dies of the failure to do so.
Theoretically, more advanced gadgetry and knowledge may lurk beyond the immediate neighborhood, and the next plateau phase thus requires an advance in propulsion tech. But we observe galaxy-wide propulsion in many near-future scenarion for the UFP. By the 30-somethingeth century, Earth may well again have run out of advances to steal from its betters, and the only way forward is going intergalactic.
Of course, there already are folks who go beyond provincial in the 23rd century, yet their benefits do not yet touch Earth. So a Prime Directive of some sort probably applies, forcing Earth (the UFP, etc) to invent ever-better propulsion all by itself in order to make contact with the betters. But better propulsion is already suggested (and is inherent in DSC!), so the potential for the plateay is certainly there, too.
Timo Saloniemi
