And in my private continuity, transwarp WAS a success, and became integrated as the new standard in the TNG era.
Not necessarily that "private". I just went over my TNG materials and back in the late 1980's there was a fanzine that somehow had gotten access to genuine Art Department materials, including sketches of Andrew Probert etc.
Sometimes these looked like a copy of a copy of a copy but there are apparently genuine screen schematics from the bridge (e.g. Picard's command chair schematics) and one of those shows the Enterprise-D
on a schematic that reads "transwarp". It's very reminiscent of the transwarp schematics from Shane Johnson's Scott's Guide to the (TMP) Enterprise
from ST IV.