The theoretical "death" of Professor Hulk could be explained by Jennifer Walters' powers in the upcoming Disney+ series, She-Hulk. She-Hulk will release on August 17th, but the few trailers that have been released for the show, along with other MCU films such as Shang-Chi and the Legend of the Ten Rings, provide many details that can be thoroughly examined in She-Hulk. Most notably, the Hulk is seen in Bruce Banner form in the post-credit scene of Shang-Chi and then again is back in Professor Hulk form in the trailers for She-Hulk.
In Avengers: Endgame, Hulk is left with his arm in a sling due to the gamma radiation explosion that happened by him snapping everyone back into existence with the Infinity Gauntlet. Bruce was still in Professor Hulk form at the conclusion of the film, so it's a major shock to see him back as Bruce Banner during the events following Shang-Chi. Bruce still had a sling on his arm in this scene, but the burns on his arm seem to be almost completely healed. By the time Hulk is seen again in the trailers for She-Hulk, his arm is totally healed.
It's unclear if it was just time that solved things, but She-Hulk herself could be the reason for this change back into Bruce and then back into the Hulk again. Her powers may be the catalyst for Bruce ending up in his Professor Hulk form for good. Both in the trailers for She-Hulk and in the comics, Jennifer Walters is proud of her powers and doesn't hide them away. She embraces her new gifts, possessing a form somewhat similar to that of Bruce as Professor Hulk, where she can have all the strength and power of She-Hulk, but still retain her personality and intelligence. This could cause a revelation in Bruce to lead him to accept his powers and accept his relationship with the Hulk, creating a dynamic where he is at peace with his green counterpart.
In the films where Bruce is Professor Hulk, it seems as though his powers were a little nerfed, which is fine for day-to-day life but not so great when an Avengers-type threat is engaged. Seeing She-Hulk fully embrace her powers could lead him to realize that he, too, should fully embrace the Hulk. It's possible that after seeing the example his self-confident cousin sets, Bruce could come to terms with the fact that it may be better to let the "other guy" take over from time to time, essentially "killing" his merged Professor Hulk form.
It's also worth noting that in Jennifer Walters' comic book origins, it is a blood transfusion from Bruce that gives her the ability to transform into She-Hulk. Maybe Bruce simply needed to be back in human form in order to give her this transfusion, as a needle would not be able to penetrate the Hulk's skin. It's more likely, though, that after the events of She-Hulk, Bruce/Smart Hulk comes to terms with Hulk and develops a more symbiotic relationship with him, as opposed to his current form where he hides the Hulk altogether.
She-Hulk: Attorney at Law premieres August 17 on Disney+.