You know there's a reason even stuff like Mega Charizard Y would not resort to Blast Burn despite the greater KOing potential. Leaving yourself wide open whether you KOed the target or not is hardly a worthy trade off. Oh, saving Hyper Beam until it's almost dead? Are you saying you have only 3 moves to work with until you're ready to kick the bucket? The only way Hyper Beam won't give up a free turn is if you did not KO the opponent and they KO you.
So you're telling me I have this awesomely strong attack, but the only way I don't risk screwing myself over when using it is if I go out in a bang but not KO my target anyway? Yeah, Hyper Beam is not good.
You have to nuance "free turn". If you don't take that "free turn" to kill Greninja and instead setup, you're wasting a kill. Ofcourse you have to take the recharge turn into account with Hyper Beam, that's why I said many times that it's ideal to use it the turn before you die to get off a big hit before going down (I think that's one of the first things I said when Hyper Beam was brought up), or to 2HKO/OHKO certain threats you couldn't otherwise. Mega Charizard Y is something completely different, it has Fire Blast which is much better than Blast Burn in every way. Greninja has Protean to make Hyper Beam usable, and it has good neutral coverage and great power.
TeraVolt I never realized you could run five Hidden Powers, U-turn, Extrasensory and Grass Knot in one moveslot. Sorry for missing that. To be serious, you're just proving that Hyper Beam is viable in the last slot by your calcs.