I think the assassinations should be necessary in Infinite's new sandbox.For example overshielded players shouldn't die by simple back smack but be assassinated while they still have overshield.
Also for Campaign it would be a nice addition that the stronger enemies don't die so easy by just punching them once or twice.
Assassinations are cool but they would negatively impact the Dance or the golden triangle,
Grenade Gun Melee, the core of Halos gameplay Assassinating is Flair its something you do to really rub in your kill but it leaves you as a player extremely vulnerable in high octane situations. So I have to disagree a melee to the back is sufficient to kill in Halo as it always has been a quick method in taking out opponents quickly. There is even a toggle in Halo 4 5 and I believe reach where you could turn of the animation in favor of a classic approach to melee combat and is something done by even pro players, as the amount of time it takes to perform an assassination is enough time for a player to make a call out and have you the person performing assassination get teamed on by the approaching enemies made aware by your flashy antics.