^^^ I like the C-130 analogy for the Miranda. That definitely fits.
I also agree with the refit statement above, although I wouldn't attribute it to "propaganda", per se, but as others have postulated, a test-bed for new components that would eventually be used to build fresh new ships from scratch. I agree that the original 12 (many of which were lost in the line of duty by TMP anyway) mostly retained their original TOS-like configuration and Enterprise was chosen for refit and may have been the only one. All future ships we refer to as a "refit Connie" (like the Yorktown/Enterprise-A/WhateverTheHellItWasCalled) were likely built new and never from refits as, yes, I would agree that would require an impractical level of effort. Thus, fully explaining a completely different interior layout, like the bridge module, crew quarters, torpedo room, main engineering and shuttlebay.