If they knew that they wanted to "end Shepard's journey" with ME3, and if they knew they wanted to (or "had") to kill Shepard at the end of Mass Effect 3, then why bother creating one specific ending with Shepard gasping for air while covered by debris. That scene could have been extended, showing Shepard trying to breath but ends up dying anyway, at least then it'd have been crystal clear that he or she's dead. But... NOPE... nope, instead they create that scene to start with, and then before anything is clear it cuts and Shepard's fate remains "up in the air" for us to interpret as we see fit. Oh, he/she COULD have survived, it MIGHT have been indoctrination, he/she MIGHT have been hallucinating part of what happened. Sure... MAYBE...perhaps... probably.... it's possible... you know what? Just make your own ending, we at BioWare are too lazy to make things clear.
Why bother giving us a 'false hope' when, internally (in the minds of the writer(s)), it is clear and acknowledged that Shepard is effectively dead since it was "the plan" for ME3. That, BioWare (well, ME3 team, specifically) was cruel. If you wanted him/her dead, then leave him/her dead! Don't spend time creating an essentially useless scene in which Shepard gasps for air, what was the point? What. Was. The. Point. Trolling us for a couple of years until ME4?
I just never understood the point of that scene, if the condition was "By ME3's ending(s) Shepard must be dead, it's canon, it's what we want, it's the end of Shepard's journey, it was a trilogy, now it's over, the end.". By the way, some people disliked ME3 in general, not just the ending(s), and not just the fact that Shepard would die by the end of the third game. I for one DID expect my Shepard to die (wasn't 100% certain, but I WAS prepared for that since ME2 was just released). Some people (like me) actually disliked ME3's beginning, many parts in "the middle", and the ending (which doesn't just including the very ending choices to take but also include the very anti-climatic London mission).