Everyone is, of course, entirely entitled to their opinion. My opinion, for what it's worth, is that it was a perfect, and appropriate way for Dean to bow out.
The first thing that struck me during that heartbeaking death scene was that this incident was perfectly placed to enable Dean to die on his feet. (This satisfied me so much, I wrote a drabble about it). Dean wasn't lying on the ground in a pool of blood with Sammy crouching over him, he was holding his own, standing up looking Sam directly in the eyes. For me, that is the single most appropriate and dignified aspect of Dean's death. However, from a practical film-making point of view, I'm sure it would have made it so much easier to clearly show that beautiful exchange between the brothers. Every word of dialogue, every nuance of expression, every tear rolling down their faces. That would have been so much harder to do with Sam looking down over Dean. The impact of that death scene will live with me for the rest of my life.
Secondly, Dean didn't go out having been beaten on a hunt. I had a brief exchange with my dear friend theymp and he articulated my own thoughts far better than I could - 'I think I would have found it intolerable if he'd been bested by some monster'. I think Dean would have found that intolerable too. He had the satisfaction of knowing that he and Sam were on the winning team when he died
Finally, the manner of Dean's passing just serves to reinforce something that is very easy to forget because he is such an exceptional character. Dean is only human. He's not a superhero (Jensen's got that to come), he's not some immortal cosmic being, he's not special. He's just human. he's a mortal man, and he's fallable. He can get sick, and stub his toe and have accidents. And that does not in any way demean or belittle Dean and his achievements; far from it. In fact it serves to show that what he achieved as a hunter and a hero, time after time was even more amazing than we imagine as we have sat and watched his and Sam's adventures week after week.
His human weakness, his fallability in the face of everything he fought and suffered makes him a titan.
Dean Winchester. Forever in my heart.