Perhaps transwarp eventually became workable and the terminology changed. I'm thinking about a time when there were airplanes and then people also spoke of jet planes, to differentiate. But today people commonly speak of planes, no longer specifying "jet" plane.
Maybe transwarp speed became the norm and people just ended up calling it warp speed, similar to airplane rather than jet airplane.
While I'm here...
Is there anything we know about transwarp that required substantial new engine technology?
The Excelsior is cited as being a new transwarp starship, as if the ship was purpose-built around a new engine.
But how do we know that the Excelsior isn't just the next great starship class and is incidentally a testbed for transwarp?
Could the new transwarp be a breakthrough but mechanically maybe just an add-on to the dilithium chamber and a new intermix formula? Or is it a total re-work of previous engine designs?
If the transwarp trials didn't work, maybe it would be a simple matter of using conventional warp intermix formula and removing the some isolated component from the engine room. The Excelsior would still be an impressive starship, not a complete lemon due to transwarp failure.