I really wish that ENT had considered taking a look at how the SFU Federation goes about its early evolution.
In that setting, there is a distinct division between the Earth ships built in the years prior to the foundation of a unified Star Fleet, and the saucer-and-nacelle designs which were built to supersede them - and the other single-race ship designs.
The development of the new generation ships not only made the racial ships they superseded (mostly) obsolete, but they were more efficient for a growing Federation to use, as they had more standardized parts for their ships which could be stored at planets and bases across UFP space.
And while major member worlds (such as Vulcan and Andoria) retained some racial ships 'to facilitate training' (or in the case of the Vulcans, for exploration purposes, since their ships were best suited for such a task) as the overall technology level of the UFP progressed, even they were mostly phased out, and the member worlds turned to older saucer-nacelle hulls as their new National Guard ships.
And in political terms, in the case of the Federation (and in that of the Interstellar Concordium, another multi-species alliance, which will have some of its pre-unified racial hull designs published in the upcoming SFB Module Y2
) the foundation of the unified Star Fleet represented a major shift in the way the allies were bound together - by uniting their chief military and scientific wings under a joint command, the founding of Star Fleet helped forge the UFP into a more closely-knit federation, i.e. by shifting more power and authority to the federal government (as the ISC Navy did for the Concordium).
(But even then, the relative power and influence of Earth in the UFP, and the durability of the Terran-design light cruiser, saw that design refitted and used in Star Fleet service for several decades afterwards - and the Police used modified Terran vessels for their purposes.)
Indeed, that distinction between the Terran-designed and unified hulls can be seen with the Republic of Aurora - a lost Fed colony in a distant corner of the galaxy, which ended up building its new ship designs based on the Terran-hull cruisers and frigates it inherited when the colony was lost, as opposed to going with the increased reliance on saucer-nacelle-configuration ships that Star Fleet adopted.