Steve Martin was a brilliant comedian and Kevin Hart doesn't stand a chance of standing next to him. The caliber of modern comedians has dropped and it's not the fault of the comedians. Americans are carefully working on dumbing down comedy in order to not think as much about what is funny.
Kevin Hart is completely overrated. He plays a buffoon well and that is all. In his movies he plays the typical idiot that becomes the hero at the end. His stand up is just a little strange and full of energy but not a lot of laughs. Martin had a long career, no one will remember Hart in 5.
I personally do not believe there is a "next" Steve Martin but that doesn't mean that Kevin Hart won't see success. Kevin Hart has already been around for a number of years and I believe he will see success with his career in the future. He's proven his ability to make people laugh and that's half of it, now he just needs some more luck.
No, Kevin Hart is not the next Steve Martin, because he does not have as broad of an appeal as Steve Martin does. Kevin Hart is very funny, and he is one of the most talented comedians and comedic actors out there, but with as fractured as media is today, it would be hard for any one to be as well known as Martin.
Although Kevin Hart is extremely funny I don't believe he is in any way comparable to Steve Martin. They have very different comedic styles and Steve Martin is a legend in his own right. Steve Martin had a long run with SNL and many hit comedic blockbusters. It will be awhile before Kevin Hart reaches that level of fame, if ever.