Because opening the Gate of Time is the safest bet for Link's quest if Demise is to be destroyed. The reason why Impa sent Link off to forge the Goddess Sword into the Master Sword is not just to open the Gate, but also have to Link's spirit grow as well.
The Triforce is not some relic that you can just touch and say "I wish". It grants the wish that one's heart desires deep down and requires the bearer to have an unbreakable spirit with the balance of courage, wisdom, and power. Without that, many things could go wrong when Link wishes for the Demise's Destruction. The Triforce might split apart with one piece that ends up in the hands of Ghirahim or someone even worse, causing for Link to hunt for them again. The trials for hero to face are to prove his worthiness as Hylia's Chosen Hero and the bearer of the Triforce. Something that Hylia would like to confirm herself.
While you might argue that Impa may confirm Link's balanced spirit and tell him everything he needs to know, that method is just simply cutting corners to the plan that will ensure the destruction of Demise and possibly risk the future of Hyrule. The death of the Imprisoned doesn't stop Ghirahim or any of his cronies to continue on Demise's legacy. New evils will arise and may possibly be invulnerable to anything except the True Master Sword. That little trip to the Gate of Time that may cause hindrances now, but it will benefit the world in the long run.
Using the Gate of Time also ensures the meeting with Zelda and give her heartwarming goodbye and promise. The last thing she sees before a thousand year slumber is Link, who promise to wake her up, and when she wakes up, the first thing she sees is Link waiting there. It's much more emotionally to give an assuring goodbye for her than to have her fall asleep, not knowing if her friend would awaken her at all. From a story perspective, Link has to open the Gate of Time and meet Zelda or otherwise the flow of the story just becomes cookie-cutter.
Not to mention that Ghirahim doesn’t know where the Gate of Time is until the last moment. Throughout the game, we see Ghirahim is searching for the second Gate of Time to capture Zelda but with no such luck. Link and Impa believe that the Gate is safe as long as no one spills the beans of its location. In fact I believe that the only reason why Ghirahim was at the Sealed Temple in the first place is because Zelda’s aura appeared there after his Master was crushed. So even if the Gate of Time is not opened, I’m sure that Ghirahim will “persuade” Link and his group to open it for him.
All in all, your alternative methods may prevent the return of Demise but it complicates the situation more than necessary and it's simply not Link if he did those things. Link always confronts his destiny head-on, never tries to avoid his fate or battles that he must face.