Avengers Hulk Theory: Does Bruce Banner Die THREE Times In The MCU?!
How Hulk Could Die...
It may well turn out that the Hulk ends up being killed off in a future MCU movie: perhaps by Thanos, perhaps by something else designed specifically to take him out, but this is all compelling evidence to suggest that he's already crossed over to the other side - at least temporarily - several times already. So even in his weakened state (or at his most defiant), it looks like it'd take something REALLY special to retire him permanently.
Perhaps it will take something like Hulk CHOOSING to die as a sacrifice to kill him permanently? Perhaps the thing keeping Banner alive is the will of Hulk to survive. He's a character who has been forced to live on the edges for so long - other than his time in control of Banner's body during Thor: Ragnarok - who fought for his right to be in that movie and who fought against Banner using him inhumanely in Infinity War. Perhaps that enduring spirit is what keeps him alive as much as his powers do?
We know there's already been a hint that Hulk will be more intelligent in Avengers: Endgame and that Banner and his alter-ego are likely to find a greater balance, so perhaps that could lead to a moment of epiphany for Hulk? If he's selflishly compelled to exist for himself, the natural end of his arc would fittingly see him realising the value of altruism, even at the cost of being able to exist. And watching him and Banner realise together that there's no choice but for them to lay down their life without a resurrection to save everyone else, we'd get a satisfying conclusion to that arc and a means to explain how Hulk could die.
For now, all we know is that he's pretty much invulnerable. At least if Banner ever has performance issues ever again and Hulk refuses to stand proud, all he needs to do is cause himself a fatal injury and all will be well again.