I don't think this is really accurate. Like, at all.
in order to TPK your party, all you need to do is kill them all. You don't need to burn through all of their potential healing. That's a sucker's game. You take every single one of your monsters, and you nuke the guy who can fire off the most healing. Once he's unconscious - and this part is important - you kill him outright. You don't leave him bleeding out on the ground. You chop his head off. You don't allow for the possibility of him standing back up. Now it doesn't matter how much healing he had left. He can't use any of it, because he is a corpse. Rinse and repeat for the next five party members.
tl;dr - Don't give your party the chance to heal. Kill them all outright.
What you say sounds good on the surface, but you forget (or didn't know) the fact that the PCs outnumber the NPCs in every encounter here. In fact, I "broke" the rules a bit and had 5 foes in the last encounter, but it didn't matter. The PCs win the action economy part of the game before the encounter even starts (a major mistake on the part of WotC when setting up this challenge, higher level monsters do not make up for number of actions per round).
In order to do as you suggest, NPCs have to at least down a foe (which I tried with NPC focused fire, but it didn't help).
Every PC had around 40 hit points remaining except the Swordmage who finished the fight with 6 hit points. The Companion was full up at 26.
The reason the Swordmage was at 6 hit points (after taking 130+ after reductions, probably close to 155 or so before reductions) was because the Swordmage and the Companion were the only two PCs that most of the NPCs could target in the last encounter. Two of the five last NPCs were Soldiers with melee only attacks, so they were totally unable to get to the PCs in the back.
I was attempting to focus fire on the leaders, but they were hanging back in the corridor behind the portcullis where they could get a cover bonus (sometimes behind other PCs) and the NPCs couldn't get to them. The BBEG did manage to recharge his area attack once and blast multiple PCs twice back there (ha ha, no cover bonus), but he also often had two melee PCs on him so that it was tough for him to get away to do that.
The Bard had used Stirring Shout on the BBEG, so every time a PC hit him, that PC got back 4 hit points. Some PCs actually had more hit points at the end of this fight from this power alone than they had walking into this fight.
The Swordmage had the Rose King's Shield feat, so she was gaining 5 temp hit points (and sometime 10 temp hit points) at least every other round. She was a defending Swordmage and saved her Encounter powers for the last fight, so she had at least one foe marked every round, sometimes as many as 3 NPCs, and if some of the NPCs would have tried to range attack a Leader, she would have stopped 7 of the damage of the attack.
The PCs had 7 healing potions remaining (2 on the swordmage, 1 on everyone else). It only takes 2 minor actions to drink one and gain 10 hit points. I was surprised that the Swordmage didn't use hers, but she said that the Leaders were healing her a lot, so she didn't see the need. Even though she went down to single digit hit points a few times, she felt confident that the Leaders would keep her alive (maybe a poor assumption on her part, but she did take more attacks than anyone else, so maybe it was a good strategy after all, she'd still have 2 healing potions remaining after all of the rest of the party healing was gone).
Using up healing surges was irrelevant. Players were going to suck down every bit of healing that they had before they would allow any PC to go or stay unconscious.
In order to kill a single PC outright in the last encounter, the NPCs would have had to go through 40+ (not including reductions) + 26 (bloodied, 32 in the case of the swordmage). Even if every NPC hit a PC enough to make him or her go unconscious, there's no way that the Leaders would have let that PC stay on the ground long enough for another NPC to do a Coup De Grace. The NPCs just didn't do enough damage and have enough turns per round to manage that. Action Economy just didn't give the NPCs enough of a chance to do that.
So yes, my players are very smart and have been playing 4E since it came out. They used cover, distance, specific powers, and other good tactics to prevent the very tactic that you suggest. They optimize their healing so that every ounce of healing they possess will be used before a single PC can be killed. They prevented the lower defense PCs from even being in the fight and forced 3 out of the 5 NPCs to focus on the defender and the companion due to terrain.
Your suggestion sounds good in theory, but there's no way that it would work that way in practice. My group would never make that rookie mistake and allow it.