Yet every structure surrounding the Cathedral is on the old Highborne style. between choosing:
-The implication of the structures right outside the bubble looking 10k years old, as well as the other structures in Suramar as a zone and the Broken Shore.
or
-The Cathedral reusing the Nighthold aesthetics because it was easier to come up with a unique tower to add the fel.
I'll chose the former.
Regardless, it's also possible that the Nighthold and the Cathedral had a more uniquely Suramarian style even back then, but the implication is clear the rest of the city would have looked more like the Highborne Architecture of the times; which again, are actually close; the thing is that in modern Suramar there are also things that would appear newer.
Basically; Every set of ruins -updated with graphical advances- has shown us that the Highborne Architectural style was rather homogeneous, that shared an overall aesthetic. Suramar looking so different wouldn't be in line with this.
The idea that the city simply changed aesthetically (as the Shal'dorei did) makes a lot more sense.
Suramar doesn't look "better" or "fancier", it just looks like 10k years passed and fashion and aesthetics changed.
Personal Opinion: Suramar is beautiful, but errs on the side of
tacky, of too much going on, of a city forced to build on top of itself.