IMHO, I don't think the Miranda was meant to compete against the constitution, other then sharing common parts. The ship was built as as a workhorse, not a front line explorer. I don't believe there were many Connies built. Supposedly, there were 12 Connies in the Fleet at the time of the O.S. I believe Enterprise was the only one that was given the "Refit" treatment. (It would literally make no sense to replace 90% of a ship unless you were doing it as a propaganda move.) and "refit" Connies after this were built from scratch. The old Connies were retired. I would guess Less than 10 Refit Connies were built in Total because The Excelsior came right on the Refits Heels. Meanwhile, the Miranda as a General workhorse ship, and a master of no trades, solders on and continues to get built. The miranda is the c-130 of starfleet.