,,,Of course, in real life, this would be something of a problem because the Excelsior was established as a commonplace background vessel and something of a workhorse, whereas the Sovereign was a hero vessel and TPTB would thus have been reluctant to "dilute" the heroic value by presenting the newer type in greater numbers or in background roles.
Timo, the other problem with viewing the Sovereign as a replacement for the Excelsior, as opposed to the Galaxy, is that they all seem to be capable of filling similar roles — they are large, multi-mission capable explorers and heavy combatants (adjusted for their own eras, of course).
But as you correctly note, by the late 24th Century, the Excelsiors had obviously become a workhorse. If Starfleet sought to replace them en masse by that point, they'd probably design a ship that filled the light cruiser (combat) and general support (peacetime) roles. I see the Nebula and Intrepids as more logical candidates for the ship that will replace the aging Excelsiors by the late 24th century than the large, advanced, heavy-combatant Sovereigns or Galaxies. Excelsiors are just too far down the duty rung to warrant such enormous "replacements."