Why? Besides the ambiguity to blue and green endings (assuming you don't have the Extended Cut, which you should have), they were good endings. If you have Extended Cut, you've got a great ending. Or did you somehow expect it to be different?
Technically speaking, Shepard would've somewhat survive through the Shepard VI. Since Synthesis requires Shepard to sacrifice himself to infuse his very essence into everyone and everything in the galaxy, it's highly probable that the Shepard VI would've become fully sentient with Shepard's persona, therefore allowing Shepard to survive in that form.
You were specifically warned about the fact that you'd have to sacrifice yourself. I understand the information was pretty sparse before you actually chose, and the ramifications were only really explained afterwards. I actually liked that. I thought it was humbling to still be part of something bigger than I could possibly understand, even after becoming possibly the most important human to ever live. It's probably for the best my Shep died after Destroy. He never would have forgiven himself for EDI and the Geth's deaths. He was only human, and made a mistake, too intimidated by the idea of taking the kind of power that Control provided, or changing the existence of every living sentient. Anyway, why would you want Shep to live through all that? I thought death was a fitting end for him/her. The ony way the Commander would ever stop fighting, the only way Shepard's story could end.
The idea behind it left a lot of loose ends. Like for instance how there were exactly three paths, one leading directly into the beam of the Citadel, one to the batteries (which were exposed, presenting a clear hazard) and one to the controls themselves. All three paths were clearly hazardous to any life form, and any result could be viewed as a short term solution.
Destroy would bring casualties in the form of all synthetics with the Reapers included. While humanity and the organics rebuild, it is possible that some of the more resilient synthetics could survive and recreate the Reapers' destruction of organics.
Control allows Shepard to control the Reapers by taking the form of a V.I. made from his mind; however, this fails to work mainly due to the fact that over time, the Reaper's logic would take the place of all that Shepard was, and the cycle begins again.
Synthesis would merge all living things at the cost of genetic diversity leaving a massive scar on the face of the gene pool (no pun intended). Additionally, while the Reapers would end up without a purpose, eventually organic life would return in some form after the genetic scarring, giving the Reapers new purpose.
And to top it all off, if you choose not to use the Citadel, you get rewarded with the Reapers taking down an Alliance vessel.
While a nice sentimental thing could be said about the necessity of sacrifice, the ending could have been done far better. Plus, I wanted to see Shepard build that damn house on Rannoch.
Everything you mentioned as consequences of the three endings were entirely possible (ok, except the genetic diversity problem. That's not how genetics work, even given the sci-fi premise of making people semi-synthetic), and that's what I love about the ending.
You say there were loose ends, and that's true, but you saw it as a bad thing, where I consider it a natural consequence of such a large story. It was only the war and Shepard's life that were over, nothing else. Defining how everything continues after that point is not only burdensome, but unnecessary and limiting. With more options, the story is left up to the player's imagination. Perhaps that's like taking the coming-home out of the Lord of the Rings, but it was a choice the authors made to free up future projects. That kind of planning has real-life ramifications.
About the house on Rannoch, though... I'm sorry. I feel your pain.
Outstanding work! Indeed, this picture menages to convey a very powerful message.
However, I don't see any reason for pessimism. At the end of the game itself, she is NOT stranded anywhere and you ARE DEFINITIVELY, POSITIVELY alive (battered and severely wounded, of course, but alive) if you take the right decisions. Therefore, it is only a matter of time before the two soulmates reunite once time for all; and this is going to happen, even if we are not allowed to see it on the scene (a pity, I think). Obviously, the player can always choses to not pick Destroy, but that is a completely different situation.
i imagined before some fanfiction, how he came back on Rannoch as a catalyst-like ghost, to tell that he will love her to the end of the universe and only their love helping him focus to help others :3