Introducing Professor Hulk in Endgame was completely nonsensical. It erased Hulk's character completely, and seriously warped Bruce's, without even any proper exploration of how it came about.
The way the 'merge' was presented is not an acceptable follow-on from the actually quite interesting dynamic developed in Ragnarok and Infinity War. If they wanted to use Professor Hulk as the resolution to Bruce's arc, there is actually a perfect alternate opportunity they missed.
The un-Snap.
It was established that most of the spill-over power of the Stones/gauntlet came in the form of gamma radiation. Hence why Bruce was chosen, having the best chance for surviving it. I found the canon version of this scene underwhelming, lost in the impact of the rest of the movie. But Bruce's Snap easily could have been made as character-defining a moment as Tony's.
Because, imagine it is a human Bruce who puts the gauntlet on. The power surges through him, burning, overwhelming the latent protection in his altered blood. But Bruce has a greater defence. Hulk.
Hulk's purpose is to protect Bruce from harm, whatever its source. He couldn't fight against Thanos, so he tried Bruce's approach instead; he hid. He hid all through the battle in Wakanda, all through the years after the Snap. But this is a new threat, and the power is burning through him as well. So he comes forward to fight it.
But there is nothing to smash. There is the gauntlet, but they need to use it, not destroy it. More to the point, Bruce needs to use it, to direct the power and focus on everything they want returned; he can't retreat and leave Hulk to fight as he has done before.
So for the first time, they truly work together - Bruce's mind and Hulk's strength - channelling the power of the Infinity Stones. They Snap.
And as the power surges through them, it finishes the process the gamma-accident started so many decades ago. They are no longer Bruce and Hulk, but Bruceandhulk.
