i agree with most of the points on the connie, including the thin spots that could be easily damaged. compiling everything we've learned on the connie and connie refit: conceived in the late thirties, laid down between 40-45, beat to death in the fifties and sixties, refit and reissued in the seventies, and not retired till the ninties (when they worked all the kinks out of the excell). the connies have, more or less, 50 years of service in.
the ent herself was in service basically from 40-85 1 mission under Bob April, 2+1 year under Chris Pike (with refit), 2 under Kirk before promotion, ALSO with refit, then 1 more, then the destruction at the end of SFS.
I think the Amby's were meant to be a one-shot ship, and the only loose models they had for next G besides Galaxy were Excells (which didn't go into service officially till 90 or so)