I've read a number of posts related to why would Destiel ruin Supernatural, and since this is the latest, I'm going to respond to this one.
First of all, yes, I am a Destiel fan, and no, it doesn't cloud my opinion, I'm trying to look at shows from a different POV when I criticize or praise them. And trust me, I'm watching a hell lot of shows. I'm not a fan of Leonard and Penny, I always imagined Penny with Sheldon, and yes, I read the fanfics about the two of them, as well as having written some of my own, but I still believe Penny and Leonard are in for the long run and that they, as well as Shamy, are better for the continuity of the show. Having Penny and Sheldon together would just ruin the show, but I'm still be reading my P&S fanfiction and awe on their moments together.
So, having written a short introduction to my thought process, let's finally move on to my favorite show of all time and my ship of the ships. I started watching SPN in the spring of 2013, even though I watched it many times before on Croatian television (I'm from Bosnia). I've known about the Destiel fanbase, but I wanted to see it for myself. And wow, right into that fourth season, from the very beginning, I could see a potential. And I don't see them as two men together, or a celestial being and a human, angel and a hunter. I see them as two souls bonded, having a special connection. Writers continued adding a little fuel to the spark, it became a forest fire, but in its very core it was still a long, complex love story between two soulmates and nobody can convince me otherwise. I do not like the heteronormativity or the queerbating present in the show. Amara and Dean have had the 'profound bond' talk and she kisses him a few episodes later, when Dean and Cas had the mention of the profound bond nearly five seasons ago, and still nothing came out of it. I'm sure if Cas was in a female vessel, they would have done the do a long time ago. Hell, if Hannah stayed alive, Cas and her would probably end up kissing each other again, but she had to be killed, because, you know, she's in a male vessel.