For all of the speculation and all of the unknowns around Avengers: Endgame, we can be certain of one thing. Someone is going to die. In actual fact, when the dust settles again, the list of the fallen will probably be significantly longer than that.
With so many MCU veterans coming to the end of their contracts and persistent rumours that Endgame will be a real torch-passing moment, the question now is how many of those OG characters are killed off. There is, of course, the possibility that some of them will retire valiantly, such as Clint Barton, whose arc very much depends on the safety of his family, or Bruce Banner, who ultimately seeks peace beyond anything else. Thor could retire too, since he has New Asgard's rule to concern himself with.
But the two characters we have to be most worried about are Tony Stark and Steve Rogers. They are the most precariously positioned of the survivors of the Decimation, not least because Chris Evans and Robert Downey Jr are the most open about their impending retirements but for story reasons.
The more you look at it, it seems likely that we're going to lose at least one of them to the second war against Thanos. And unfortunately, it makes sense that, like Harry Potter and Voldemort, neither can live while the other survives. Marvel can't just leave them both to walk off into the sunset, so something has to give, and the most likely eventuality is that Rogers lays down his life to save Tony Stark.
To start to understand that, we have to first look at the characters in more detail. That way, we can deal with why one of them has to die and why it ultimately HAS to be Captain America...
4. Tony Stark & Obsession
In many ways, Tony Stark and Thanos are mirror images. Thanos even acknowledges their similarity in his parallel that both are "cursed by knowledge" and his admiration for Stark does seem implicitly tinged by recognition.
Tony absolutely has the potential to be JUST like Thanos, which is both the most concerning thing for Tony's future and the key to Thanos being so perversely sympathetic. Both talk in terms of extreme solutions for "the greater good:" for Thanos it was the Decimation (having played with the idea of random genocide to save Titan). For Tony, it was creating Ultron and then endorsing the Accords to regulate superhero activity.
In both cases, they planned to consciously ignore the negative impact for the sake of the longer term positives. They appointed themselves messiahs: Thanos in a more maniacal fashion, but there's no doubting that's how Tony saw himself for a time as well. Why else would he break protocol and logic to announce himself as Iron Man?
The problem with that complex and with Tony's knowledge "curse" is that he cannot simply retire from being Iron Man in conventional terms. He's tried it and it didn't work, to the point that it drove a wedge between him and Pepper Potts. Think about how his need to be Iron Man defines him in Iron Man 3: he's presented as an obsessed genius, endlessly creating armour variants to fill the whole inside him. In place of sleep and self-care.
For him, being Iron Man is an illness, which is why his being a superhero has been tied so closely to illness - both mental and physical. It's no accident that his arc reactor was both the thing keeping him alive and killing him (thanks to palladium poisoning). It's also no accident that his PTSD made him fearless in the pursuit of protecting his loved ones and the planet, but also destroyed his mind for a time. He is a man defined by conflict.
If he cannot walk away, even with the promise of his future with Pepper and their marriage and potential children, his only way out is death. Unless Captain America changes the parameters, that is...