Why would they quit building their workhorse class (Constitution), when their envisioned work horse hasn't even been through trial runs yet?
There's nothing to indicate that the Excelsior
would have been intended as the new workhorse. Quite to the contrary...
class being dropped from the exalted initial position indicated in ST3 to the status of workhorse as seen in TNG may well have taken the entire intervening half a century.
And if the Excelsior had already been flying at incredible speeds, why would hey abandon the project because of a little sabotage?
There's no evidence that the project would have been abandoned. For all we know, both the ship and the drive were splendid successes.
Both TNG and Voyager discuss the Borg having Transwarp drive in almost magical terms. I'd think it highly unlikely they'd so this if StarFleet had developed a working Transwarp system within living memory.
But transwarp probably just means "better than warp". There would exist a transwarp drive at any given timepoint: in Archer's days, it would be the warp seven drive, in Kirk's days it would be the Excelsior
drive, and in Picard's days the goal would again be set higher.