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.
I think this 80 year "separation" you speak off makes any direct line from TOS to TNG not workable, IMO.
For example, what would be the TOS Enterprise's evolved descendent in TNG time? The Enterprise-D? Voyager? Or the Equinox? I'd argue the Equinox because she has the most similar features. The Enterprise-D's bent warp pylons points to the TOS Aurora cruiser that combined with the USS Excelsior
The Defiant's unique look is her engine cowlings. Take the cowlings away on a TOS-era design, I dunno, it starts to look like the USS Grissom, IMHO.