Correct me if wrong, but the Excelsior class ship wasn't built to take over at all when we first encountered it; it had the experimental registry prefix of "NX" instead of NCC. I took it to mean it was a test ship for a potential class of ships with transwarp drives. Only after the failure un-seen failure of the experimental drive, did it seem the ship was possibly brought into production. I say possibly, because the Excelsior was the only one we saw, maybe meaning that when the test failed, they decided nto to abandon the actual ship and re-classify it for standard duty. It wasn't until TNG time that we found another ships of that class.
Its actually a popular misnomer that the Transwarp drive "failed". Scotty tampered with it, and Excelsior was therefore unable to pursue Enterprise, but we see the ship again still in spacedock in the following movie (stock footage, I know) and I find it difficult to believe that the Starfleet corps of engineers would shrug their shoulders and assume the drive itself was a dud based on a single incident, especially when the saboteur is conspicuously later found alongside the rest of the crew of the escaping vessel. Just as likely is that Scotty's tampering was unravelled and that the transwarp experiment was ultimately a success, and led to the recalibration of the warp scale seen from The Next Generation onwards, reclassified as 'standard scale' warp drive (the 'Transwarp' label then being applied to whatever the next, hypothetical increase in warp capacity is likely to be.) Food for thought.
