Look at it from an existing example - compare the TOS E to the E-D. Yes, they both have a saucer, a dorsal, a secondary, and two nacelles on pylons but the ships don't look like they just kept swapping out parts over the years until one turned into the other. They're both unique builds.
I see your point. But if you were to put the TOS Enterprise next to the TNG Enterprise-D, I would have a hard time making a direct connection about the two other than a general arrangement because they're too far apart generation-wise. The E-D could have evolved from the E-C but beyond that the connection is rather weak.
The Defiant feels more like a ship that was built during the TOS Movies and got shelved. She has similar greebling as the Reliant and the same hull coloring of that time instead of the later blue/blue-green hues of the TNG and Voyager. It is alot easier to make a connection back to TOS by using the TOS Enterprise to TMP Enterprise upgrade as a working model, IMHO. In anycase, the other examples of a TOS Defiant seem all workable as well as long as they have the distinctive nacelle cowlings...
Comparing E-C to E-D or TOS to TMP defeats my whole point - they're SUPPOSED to look like they directly evolved from one to the next. From TOS to TNG era is 80-odd years - there should be more separation in ship design in there. These TOS Defiants don't have that, or at least not enough to my eye.