It could be, though, that transwarp didn't scale up well. A series of test rigs might have verified the principles, and encouraged Starfleet to build a ship that not only had a crew aboard, but also was lavishly equipped with weapons (and no doubt with other mission gear as well)! The results might have been disappointing nevertheless, with the Excelsior either not performing quite as much better than the test rigs as her bigger size and power would have indicated, or actually performing worse than the rigs.
Such things are familiar from the aircraft industry, where a subscale testbed may perform well, but an actual prototype may run into serious aerodynamic problems by virtue of being 10% bigger or having a weapons hardpoint marring the wing profile. It's just that the aircraft industry can shrug off such annoyances and simply build a second prototype; ships are more expensive...
Clearly, the Excelsior wasn't a complete failure. We could speculate that her original drive system was, though, and that the system was ripped out and a new, more conventional one installed instead. But in that case, it is a bit odd that the design would go to series production without major alterations to her shape - even the original engine shapes would be retained in subsequent Excelsior class vessels.
Timo Saloniemi