It should respawn if you leave the egg in the end for a week or more, that way it is the option to respawn it. Problem solved, everyone would be happy, I honestly would like being able to respawn the dragon so I can keep building onto one awesome world for months instead of "just restarting" like many annoying people say.
An idea: a new craftable item called the Dragon Summoner. It's crafted with the dragon egg, maybe with a block(s) of some valuable resource (like diamonds) and when used in the End, the crystals and Enderdragon respawn. What if you lost your dragon egg or want to keep your diamonds? Make it available in villager trading for a hefty price. This way, those that would like to fight the Enderdragon again would be able to should they so desire, but at a great enough price that they won't do it over and over to grind XP. And if you want to build in the End and make Enderman farms? You don't have to spawn it again. You're free to do whatever you'd like with the End, and that's what Minecraft is all about: do what you want.
I like the idea, because some people like to kill him again. The only problem is, it is a little overpowered because when you kill the Ender Dragon, you get a load of XP, so people will use it as a advantage and will keep killing the Ender Dragon to get loads of XP.