Still champions like Zed or Leblanc.
Dive Fighters can function similar to Assassins in that they can home in on a target and smash them quickly. Unlike Assassins however, their main defensive tools is statsticking and face tanking their way through damage. All you're doing is trading off one trait for another. Instead of a somewhat tanky engager that can threaten the hell out of someone, you've got a squishy one shotter that will probably get nuked shortly after bursting someone else down.
It's like comparing Mages to Marksman. Yes, some Marksman can Burst harder than some Mages, and yes, some Mages and DPS harder than some Marksman. Even then, you'd still be wrong to call one unilaterally superior to the other in the other's job, just like what you're doing right now with Dive Fighters and Assassins.