Why don't people understand that Cena can never be truly a heel?

He shouldn't turn heel now, for the same reasons you gave. But to say he can NEVER be a heel is not true. He does get booed by a decent part of the crowd. But he could very easily turn heel sometime down the road, he's been an effective one before, but I wouldn't expect it anytime soon. Even Hulk Hogan became a hated heel. I've heard it said, that to truly hate someone, you had to have loved them before.