Warning: SPOILERS for Thor: Ragnarok are ahead!
Up until Thor: Ragnarok, Bruce Banner had only spend short bursts of time as The Hulk in the Marvel Cinematic Universe, but his return in the God of Thunder's threequel came with the revelation that the Green Goliath had been walking around since the end of Avengers: Age of Ultron, i.e. for two years. With that extended period came Hulk becoming more intelligent, speaking in full sentences rather than just growls and a few words. Eventually Banner reverted to his normal, PhD-holding self, but by the end of Thor: Ragnarok, Hulk was the one back behind the steering wheel, and with him returning with most of the MCU's other major heroes in Avengers: Infinity War next year, we're wondering if this time, the character's transformation will stick for good.
Upon reuniting with Hulk on Sakaar, Thor attempted to turn his fellow Avenger back into his human form with the ol' "Sun's getting real low" calming technique that Black Widow used in Avengers: Age of Ultron, but it didn't work. It wasn't until Hulk heard Black Widow's last message to him while on the Quintet that he finally reverted to Bruce Banner. Mentally, Banner was just as sharp as ever after two years of being "away," but because Hulk had remained in control for so long, the scientist confided in Thor his fear that if he transformed into Hulk again, he might not come back. It took Banner everything in his power not to get too stressed out by the craziness on Sakaar, and the Hulk parade definitely didn't help.
Unfortunately for Banner, it became necessary to unleash his darker half when he, Thor and Valkyrie arrived on Asgard to battle Hela and her forces. Thanks to Hulk, the Fenris Wolf was defeated, and had he continued fighting, he might've stopped Surtur from carrying out Ragnarok, which would have guaranteed Hela's survival. But Thor was able to talk him down, and by the end of Thor: Ragnarok, Hulk is seen with the rest of the Asgardians aboard Korg's giant spaceship heading for Earth, and that's the important thing to remember: Hulk is still in control at this point, not Banner. Meaning that if Banner is correct, this latest Hulk shift could be permanent.
So what does this mean for Hulk in Avengers: Infinity War? Well, presumably he'll still be active when that movie kicks off, especially since the bigger spaceship that encountered the Asgardians' ship in the Thor: Ragnarok mid-credits scene has been confirmed to belong to Thanos. If Thor, Hulk and their allies have been battling the Mad Titan and the Black Order, that's only going to strengthen Hulk's hold over Banner. And although Bruce Banner getting his science on has its place in the MCU, it's safe to say that moviegoers get a bigger kick out of watching Hulk smash, so it stands to reason we'll get plenty of the latter in Infinity War. But is Banner truly gone for good? I suspect not. It may take a miracle to bring him back, but I doubt that Marvel wouldn't keep Mark Ruffalo on solely as a performance-capture actor. Plus, if there's ever going to be any resolution between him and Black Widow, it would be better if Banner was human again rather than stuck as Hulk when that happens. However, there's another option to consider regarding Hulk's future.
Mark Ruffalo learned from Marvel Studios president Kevin Feige that Thor: Ragnarok, Avengers: Infinity War and Avengers 4 form a special Hulk trilogy. Ruffalo also said that Hulk being able to speak more than a few words in Thor: Ragnarok is a key part of this trilogy. While the Hulk in the earlier MCU movies was mentally comparable to a toddler, in Thor: Ragnarok, he's closer to a five or six year old. So the longer the Hulk is around, the more he matures, which potentially means that eventually he'll reach a point where he has average intelligence or even absorbs Banner's impressive intellect. There's precedence for this from the comics, like the main Hulk's Doc Green personality or the alternate future Hulk known as The Maestro. As an even more radical idea, perhaps the Banner and Hulk personalities will meld together and become one whole individual. The human Bruce would never return, but he would live on within the Hulk's body. That's quite the progression from the man who was trying to eliminate his "condition" in The Incredible Hulk nearly a decade ago.