Marvel secretly explained why Shang-Chi's post-credits scene, but his appearance was an even bigger shock. Ruffalo was shown in Bruce Banner's human form, a drastic change from the Smart Hulk form he recently gained.
For much of Hulk's MCU story, the inner conflict for control between Bruce Banner and Hulk has been a central part of his story. This saw Hulk take control at the start of this Smart Hulk form would be what he'd look like moving forward, but Shang-Chi and the Legend of the Ten Rings showed this wasn't the case. Marvel's reversal of the moment puzzled many, and now it is more clear why they did it.
Smart Hulk's MCU future after Avengers: Endgame was quite unclear, as he suffered permanent damage to his arm after using the Infinity Stones. This injury was a further sign that the monster side of Hulk could be suppressed with Bruce's personality shining through instead. However, it also meant that there wasn't much conflict for Smart Hulk to deal with. The rivalry between Bruce Banner and Hulk was fixed, and Bruce's human form was completely out of the picture. It turns out this is likely why Marvel made the switch in Shang-Chi, as the studio always wants to have Bruce Banner be an important part of Hulk's character according to an expert from The Story of Marvel Studios: The Making of the Marvel Cinematic Universe.
As Feige explains, "We never even considered doing Planet Hulk because, as cool as we thought the comic was, Bruce Banner is not a part of Planet Hulk." And Banner has always been an important component of the Hulk storytelling the studio wants to explore.
It isn't too surprising that Marvel Studios wants to keep Bruce Banner involved in Hulk's stories. Mark Ruffalo is fantastic in the role, and even though he's fully embraced the motion-capture part of playing Hulk, he likely doesn't want to be in a mo-cap suit all the time. This also opens Hulk up to be closer to his normal comic self and have more struggles with his anger. Shang-Chi reversing Smart Hulk's transformation and Marvel's interest in the Bruce/Hulk relationship could even play into the rumors of a World War Hulk project entering development.
That said, Shang-Chi and Marvel also highlight how shortsighted the Smart Hulk transformation might actually be. This was clearly done so Hulk could be the one to wield the Infinity Gauntlet and reverse Thanos' snap. There was no long-term plan in place for Hulk's story, so Shang-Chi and the Legend of the Ten Rings and Marvel's future plans apparently evolved to a point where Smart Hulk was no longer needed. While original Smart Hulk transformation seem a bit out of place in the grand scheme of the MCU.
