Greetings.
Remember that in NHB type sparring, the opponent is quite skilled and stepping back may open for a shoot and/or grappling attempt, since you chain punched high.
Actually moving forward, like in chain punching, is a much bigger opening for a takedown then stepping back. People that are constantly moving away are hard to takedown, unless you can drive them into something, like a fence.
Trained fighters react different then untrained people. If you want something like this to work, every punch has to land hard, and you got to rock them pretty good before you drive forward. Basically what Belfort did to Silva.
Fighters basically train a reaction that when getting hit, instead of trying to get away, they drop a little and drive forward. Either forcing a clinch or going for a takedown.