The "this is a dream" film interpretation is a popular one whenever there's something that doesn't make sense on the screen. I think Inarritu would've mentioned that the end was a dream in an interview somewhere if that was the case. He's been pretty open about saying that he wanted the end to be ambiguous.

But I agree that ending makes the most sense. I don't think you not seeing that is dumb-- the idea that the ending is a series of flashes before his eyes isn't firmly established on the screen, so not seeing it makes sense. It's a take that one would need to be specifically looking for. (I'll specifically look for it on my re-watch to see if it's firmly established on screen... but, again, I suspect it isn't.)

(I'm also inclined to say I shouldn't be debating the particulars of a film I haven't seen in four months, so it's very possible others will present evidence that has simply left my mind.)