VOY - Threshold
Is there anything in canon (live-action episodes/movies) that says transwarp drive was a failure?
says that the Warp 10 "infinite velocity" transwarp barrier had never been broken by the Federation until Tom Paris did it in 2372 and turned into a salamander.
However, that doesn't necessarily mean that transwarp drive like on the Excelsior was a failure, since the Borg and the Voth use transwarp and it simply operates like a much faster warp drive, and not like the Warp 10 transwarp barrier where you supposedly occupy all points in the universe simultaneously (and can also travel backwards in time according to TNG Time Squared
) and can drop out anywhere in the universe instantly.
I've just always figured that the adjusted warp scale between TOS and TNG was because transwarp was successful. All those times that the Enterprise in TOS went to ludicrous speed like warp 10, 11, 14, 22, and 36 are equivalent to the multiples of warp 9 used in TNG, like warp 9.6, 9.9, and 9.99999999, etc.