Think of it another way an old D7 beside Discovery would look stupid. An ancient looking Discovery would not attract audiences
Using an old D7 would look stupid. Using a spruced up D7 like a Kt'inga or some other derivative(See TMP, TNG, STVI, VOY, Into Darkness. I think DS9 went back to the old ones, and don't know if ENT ever showed any.) would have gotten you accolades from the fans.
I don't mind continuity errors. A setting that has lasted as long as Trek and been under so many writers and producers, such things are inevitable and coming up with explanations to make them fit is part of the fun of being a fan. But this? This isn't a continuity error where somebody forgot a detail and nobody on the production team caught it. This was deliberate. They knew D7 Battlecruisers. I can believe that
a writer or producer knew the name but not the image, but an entire production team of such and nobody bothered to even Google the things? No. This is deliberately erasing all that came before.
At least Abrams had the bare decency to make up his own timeline.