The point of the saucer-and-nacelle hulls was not to have an overtly "Terran" design heritage, the way the old CL does; but rather, to create a new, shared hull type which would be appropriate for the united Star Fleet.
The old CL was but one of many ship types in the original Terran fleet, serving alongside the planetary navies of the other founding members of the Federation (Vulcan, Andorian, Alpha-Centauran, Rigelian). The Fed POL also has an Earth heritage; the old Terran destroyer had the same external layout, but was substiantially different internally.
Originally, these ships were "sublight"; or rather, they had non-tactical warp (impulse) engines only. They could go around nine parsecs a day in transit, but had to slow to sub-light speeds when fighting. The key thing about the sublight Terran CL at this point was its sheer ruggedness; its hull incorporated a Rhodinium alloy which other member planets were slow to adopt, and its ability to incorporate new upgrades and advances was virtually unmatched within Federation space. (The other Terran ships were less successful; as time went on, they were gradually phased out of service.)
Starting in the Y60s (Y1 being the year of First Contact) the Federation adapted the W-series of tactical warp technology; refitting their sublight fleets with more powerful engines allowing them to fight at low warp speeds, adding in the first wave of warp-class weapons (phasers, photons etc) and incorporating the kind of modern technolgies (such as transporters) that tactical warp made feasible. As it happened, Earth did pioneer tactical warp in the UFP; the very first W-era ship was a Terran CL. However, to avoid the risk of political fallout from this effort (which raised the spectre of Terran domination among the other members) the WCLs were offered as being under "Federation", not "Earth" service.
Two decades later, the time came for the Y-era; the first generation of ships built from the keel up to integrate tactical warp technology. This saw the onset of the first saucer-and-nacelle ships of the Star Fleet; designed from the outset to supersede the old planetary fleets (which instead became the National Guards of each major planet). At this point, the Federation could have plumped for a "new" CL; but there were so many Terran-hull light cruisers in service (and they were so readily able to be converted into Y-technology) that it was decided to simply upgrade the WCLs to YCLs rather than phase them out.
And forty years after that, when the time came for the first "modern" ships, it was still more cost-effectie to turn those YCLs into CLs than commission a brand new set of light cruisers.
Only by the dawn of the General War did Star Fleet turn to the
Kearsarge-class new light cruiser as a replacement design, and only then because the cut-down nature of the NCL made it cheap enough to field in vast numbers.
Actually, there was a limited run of
OCAs built in the Y120s; brand-new (at the time) hull based on a stretched OCL. (The old Terran heavy cruisers had a totally different hull type.) The class was intended as a design competitor to the
Constitution-class CA; they were solid ships, but were passed over in favour of their saucer-and-nacelle counterparts. They were the only new class derived from the CL template built in the Alpha Octant.
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Possibly one batch are the refitted ships which should have the aft phasers?
