A significant percentage of the One-Punch Man fanbase still believes that Boros’ trump card leaves him on the verge of death, even without Saitama’s Serious Punch, because he says he’ll release all his energy in the attack. However, Collapsing Star Roaring Cannon (CSRC) is not a suicide move.
Why did Boros die
Boros’ CSRC was overpowered by Saitama’s Serious Punch, which created a shockwave so strong that it cleared the sky of half the globe (besides destroying a part of the ship). After being hit, we see Boros dismembered and shredded as result of the shockwave.
Due to the massive scale of Saitama’s attack and having used up his energy, Boros is unable to recover from his serious injuries and fades away (unless he’s been rescued by Blast and healed by Fubuki at the end, hehe).
Energy & Lifespan
One of the strongest arguments of the self-destruct attack theory is how Meteoric Burst shortens Boros’ life. However, the statement is often misinterpreted. The manga simply states that this form puts “a large burden on his body”, while the anime adds that because of that “it shortens his life”.
Doing this places enormous stress on my body, similar to anaerobic exercise. As a result, it shortens my life.
Meteoric Burst (and thus CSRC) doesn’t covert life force into energy, rather it allows greater output from Boros’ latent energy by straining his body physically (eg. putting his muscles under stress by overcharging them).
Boros releasing all his energy into CSRC means draining his latent energy, not his life force, which is chipped away by the straining of his body.
The “using all my energy into this move” or all-out attack is a common trope used to underline the character’s resolve and it doesn’t entail the user’s death.
Misconceptions
Boros’ injuries are caused by his own attack.
We can see him still whole after unleashing CSRC and before Saitama’s shockwave hits him.
Boros died because he had a marble that broke, like Melzargard’s ones.
There is no evidence suggesting that Boros’ regeneration is related to Melz’s. Besides belonging to two different alien species, the glow that we see in the anime is from Boros’ glaring eye, not a marble. The manga doesn’t pay much attention to it.
The eye is his weak point or “marble”, that’s why Boros bled when Saitama punched him in the chest and why he doesn’t have his eyes after being defeated.
I’m just going to call that a long reach; anything is a weak point if hit by Saitama.
The orb that broke right after Saitama’s serious punch was the core of the spaceship, and it broke due to the massive shockwave resulted by it. There is no evidence that connects the ship’s orb to the source of Boros’ power. It’d be more logical to assume that it isn’t.
The heartbeat before launching CSRC was not an indication of a regenerative orb, rather a parallel to the first episode that they added only in the anime to emphasise how Boros can relate to Saitama (masterly done in my opinion).
Conclusion
With this in mind, what I assume would happen if Saitama didn’t counter CSRC is that Boros is put in a state of fatigue, reverting him to his Released form (you know, similar to the Super Saiyan 3 transformation, at which Meteoric Burst definitely doesn’t hint…).
Boros still managed to survive briefly the Serious Punch’s shockwave after it overwhelmed his attack, meaning that he still had energy after using his trump card, but not enough to recover from Saitama’s trump card.
