Dale Sams wrote:
"Memory Alpha and common lore seems to hold that "It just didn't work" or "The dilithium broke down"
That doesn't make a lick of sense. You would never get to the production stage where Excelsior was if the damn thing didn't even work.
My personal fanwank was that what they are calling transwarp drive is simply TNGs warp capability and not what the Borg had. Makes sense to me."
Your intuition serves you well. When I met Andrew Probert in 1988 at the Star Trek Art Department he confirmed what you just feel. By the time of TNG they have simply dropped the "trans" prefix.
Another canon hint comes straight from the bridge monitor displays of the NCC-1701-A: All the engine displays read lout and clear "Transwarp", so indeed, the evolution of warp technology did happen...