I don't see any reason why Starfleet would want to wait for a A class ship ship named X to be decommissioned before launching a ship of B class, regardless of whether this is named X or Y. Starfleet is not in the business of launching names. It launches ships. If Starfleet needs a ship of B class, it certainly won't stop to wait until some other ship liberates a name for use.
If a Nebula
class ship was ordered under a 61000 range registry, it is
possible she would only be launched after quite a few 71000 range ships left the slipways. The delay in launching her could be due to all sorts of factors - but waiting for a name to get liberated would not be among those.
I don't dismiss what I don't find inconvenient as easily as you do, Timo.
Well, it's more a case of me automatically dismissing everything until it can prove to me it comes with a canon pedigree.