To paraphrase both Shadao and Random Person, the going-back-in-time business really isn't a cop-out, but it's far from simple. The lack of explanation for confusing plot elements isn't inherent to Skyward Sword; almost the entire Zelda series is guilty of some sort of unclear definition (the Temple of Time seeming to jump around from location to location being one point). Skyward Sword just decided to end itself on the note of time travel, which is probably not the best time to wrap things up if you're not planning on explaining everything.
But as Shadao said, Link was not sent to the past specifically to defeat Demise - at least, not until Ghirahim made it clear that doing so spelled death for them all. Zelda went to the past before them for two reasons. One, she needed to escape from Ghirahim, who very nearly caught her until she and Impa went back in time, out of his reach (until he discovers the second Gate near the end of the game, of course). Two, after revealing key information to Link when he caught up to her, she would seal herself in crystal to prevent Demise from breaking out of his own seal early. In the present, you can actually see the crystal Zelda is sealed in behind the doors where old Impa sits, before you even talk to her. This is where Nintendo should have sat down with the player and explained why Zelda has already sealed herself if Link hasn't actually caught up to her yet. If he has, then the events of Skyward Sword should already be over, and Link continuing his journey is pointless. Perhaps it simply means Zelda would have sealed herself if Link hadn't come and talked to her first.
Going on, Zelda was only in the past for her own safety and to preserve the seal on Demise. In the present, Link squashes Demise like a bug and wakes the crystallized Zelda up - still in the present. Ghirahim seizes the opportunity to kidnap her and enter the Gate of Time to resurrect Demise in a previous era, and it's only now when Link must go back through time. Technically, the game wasn't supposed to end with Link returning through the Gate of Time. He was supposed to kill Demise in the present, and that was that. The world as we know it is saved. Ghirahim's plot was to take Zelda to the world as we didn't know it and continue his schemes there.
It should also be noted that in the past, Demise was only just sealed. Killing him then would kill him in all the future eras as well, and there would be a very small window to go back to in order to find him still alive, if it was at all possible - the Gate of Time may only take those who enter to certain eras rather than wherever they wish. You'd also be hard-pressed to find someone who both remembers Demise's existence and is willing to resurrect him, considering Ghirahim is gone.
All in all, the time travel itself was not a cop-out. It made perfect sense on its own, and Nintendo knew what they were doing. The side-effects caused from the time travel were cop-outs, and Nintendo's guilty of cop-outing through the whole series. It's one thing to use mystery as a strength, but another entirely to refuse to fill in necessary gaps because you want the fans to do your share of the work. I must agree with Random Person's previous viewpoints in that Skyward Sword raised more questions than it answered - even "solutions" such as the Master Sword's origins bred some new problems, such as how it was said in A Link to the Past and Twilight Princess that the sages of old crafted it whereas it was all Link's doing in Skyward Sword. Nintendo makes a lot of cop-outs in the series, but the time traveling aspect of Skyward Sword, on its own, is not one.
