The Vanguard series of novels already has a TOS-style Miranda class in operation, the Bombay. Some fan-art has shown her with a more simple hull design (like the TOS E) and TOS-style nacelles. I find that highly plausible
OTOH, might be that the TMP style of nacelles was originally launched in the 2260s and was installed on all Mirandas from the get-go. Refitting these modern things to the ancient Constitution, a tub so old that Starfleet had skipped ever refitting the boxy 2240s-50s DSC style nacelles onto it, would have been rather tricky and resulted in the need for extensive trials and lots of nail-biting in the movie, even if the engines themselves were already in service elsewhere.
and with DSC set before TOS, Magee's still flying around during TNG isn't something I find odd.
But the Magees and Zimmermans in the post-TNG short weren't supposed to be "still flying around" - the story implication was that they were being built from scratch at that late date to fill an acute need. (That is, unless the camera pointed at a rare dock where refitting instead of building was ongoing. After all, the pictures themselves did not show any construction in progress, only fully completed ships.)
Why Starfleet would choose to build to old blueprints, we don't know. But they may be doing that with the Oberths in the mid-24th century, too - and NCC-1701 may herself have been built in the 2240s to old blueprints harking back to the NCC-1017 era when round nacelles were still the height of the fashion.
Timo Saloniemi