I don't really see the Ambassador as the successor to Excelsior. More like a stablemate, a contemporary, a battleship (or, as Starfleet so nicely puts it, "explorer") to Excelsior's workhorse heavy cruiser. Every generation of ships would supposedly have those, including the TNG one (Galaxy to the workhorse Akira) and the TOS one.
Except the "TOS generation" would really be dead by the time of TOS, only Kirk's ship and a few others surviving as relics. And the respective top ships of each generation would be not just much less numerous than the workhorses, but also far more short-lived, due to being chief champions in the rat race. So we never got to see the kingpin ship to the fleet that left behind Kirk's little ship for us to admire.
Would there be a generation in existence during TOS (that is, during the 2260s), one more modern than Kirk's relic? If so, it would probably be the one that begat the Miranda and, as its last hurrah, the Constellation. But only fandom has produced big ships for that era.
Would there be a generation in existence before TOS but after the "TOS generation proper"? If so, this is where the Abramsverse ships would fit. Plenty of candidates for kingpin classes there... And fewer candidates for workhorses!
Would a generation exist between the Excelsiors and the Galaxies? If so, this is where the dark and angular Steamrunner and Saber would fit. Neither of them would be the kingpin of that era, but if Ambassadors are basically gone by TNG, then the dark and angular kingpins have an excuse for absence, too; random factors just make that latter absence more complete than the former.
Of course, even kingpins might have peers. The Galaxy has the Nebula, another relative giant. The Ambassador would be entitled to a similar peer, too (and as we know, Sternbach almost gave us one in "The Pegasus"). These would be distinct from the peers of the workhorses, which also would tend to come in nacelles up / nacelles down pairs. But on screen, we already have the Niagara, which is an Ambassador peer sizewise, uses the same secondary hull, and alluringly comes in two batches of registries, the former of which might have sported "original" Ambassador style nacelles where the higher-registered batch is seen mounting Galaxy style ones.
Basically, the pegs fit, even without a Mjolnir for assistance. We'll just have to wait and see whether TPTB dare give us a ship that would be kingpin to Sovereign (which really is a fairly modest ship by dimensions) or to the oddly homogeneous workhorse stampede of PIC...
Timo Saloniemi