On the one hand, I totally see that. For as much as WWE loves pushing the narrative that being a WWE superstar is the only dream their talent has had since they were in the womb, fact is there's probably a good number of men and women in the back who are just fine showing up to TV for their occasional jobber match or to sit in catering, collect their pay, and keep it pushing. Happens in every profession no matter what it is. For some, the time to work hard is when you're trying to get to the dance; once you're there, just kick your feet up and enjoy the perks.
At the same token, talent aren't the ones booking the shows. They're not the ones deciding who does and does not get TV/mic time. Many of them have almost zero say in shaping their promos or characters. Cody may be right in that Vince and co don't hold anyone back, but they also don't afford everyone the same chances to improve or advance in their career. You're not going to tell me Apollo Crews as had as many chances as, say, Baron Corbin or Elias. Not to mention NXT talents who get called up and are almost immediately jobbed out (i.e The Ascension), thus killing the crowd's investment in them.
Rusev is a perfect example of an NXT alum who has proven himself time and again since being called up, holding his own with and even outshining top talent, and he's stuck in the midcard for reasons no one can explain. I don't know much about the man when he's off camera, but based on what I see when he's on my screen, I'd be hard pressed to say he is where he is because he's lazy.