I think I made a slightly different point, WRT Texas -- if the state DOT formally adopts a roadway segment (such as by TX Transportation Commission order?) as an Interstate route, after getting any necessary AASHTO and FHWA approvals, we needn't wait for signage. But the state needs to affirmatively follow up on the Federal-level approvals, even if not necessarily (but ideally) by putting up signs. That avoids a situation like US 90 relocation in Beaumont TX where the DOT gets approvals, then decides "oh, never mind".
So I see another potential distinction here: relocation versus extension.
If a route is currently signed as going one particular way, it makes sense to not jump immediately on paper changes due to the concerns mentioned above: the possibility exists that the DOT won't actually bother to change the signs in which case what the signs say continues to govern.
But if a route (or part of a route) is simply not signed currently, then this isn't an issue. So there is no reason to hold off on connecting I-265 because there is no existing different signed routing and therefore no risk that IN and/or KY will ultimately decide to leave it signed elsewhere.
And I would favor inclusion of I-69 in Memphis and the I-555 extension for the same reason, provided that they both in fact have AASHTO and FHWA approval (sounds like the latter may not have okayed one of them yet?)