On many key topics Melenchon has more in common with the FN than he does with Hamon and is too much of a narcissist to give way to Hamon anyway.
Oh, please, stop with that argument. The FN uses part of the historical left programm, not the other way around. With that kind of thinking, you could say that 1981 Mitterand is close to Marine Le Pen.
Also, it's not about Hamon, or narcissism. It's about the PS. It's Hollande who waited too long before saying he wasn't running again, it's the surprise victory of Hamon during the primary while half of the party is leaving for Macron to keep with Hollande politics, which is exaclty what left voters don't want.
Yes, it's a mess, but it's not about anyone ego.
Melenchon is polling about the same as in 2012, meaning his base is still here. The PS score went from 29% to 12 for Hamon, because a huge chunk of it went for Macron, who also snatched most of the centrists and some of the right.