The "average person" (which I assume means muggles or near-muggles) would die because the minimum ki/life force requirements are more than their bodies have. Just like the Evil Containment Wave, this can be avoided (or at least greatly mitigated) by simply having enough power. Tien uses it rather casually when he fought the Frieza Force so his power has probably increased to the point where the amount of life force the attack takes relative to his total life force isn't that great.
Something seems a bit wrong with this logic, but I'm not sure what. Also, I may be wrong but... I don't recall a battle like that. Was it in the middle of GT (which is the part I didn't watch) or did it happen in Super? Or was it actually in Z and am I just misremembering it? Mind telling me where it was?
My point: It needs a lot of damage, but only in terms of absolute damage, not relative damage. If it requires 10k HP to use, that's deadly for someone with only 10k HP, but less so for someone who has 1 million HP.
So, say that a a high-power person has 1 million life force units spread throughout their body. A person's blood makes up 7% of their body so assuming blood holds the same amount of LF, that's 70k units of LF in the blood. IIRC, a person can lose 10% of their blood before suffering from any debilitating effects, so that's 7k units of LF free to use.
So, 0.7% of the body's LF can be used all at once without any debilitating effects due to blood loss and with life force loss localised only to the used blood instead of the entirety of the body, sparing the user of a loss in lifespan. Whether or not 0.7% of a person's LF is a lot depends on how much of a person's LF is taken by the Tri Beam, and that's something I don't know.
Ah, there's where I don't think the idea adds up. Now, maybe it's just me, but I think you might have it a little backwards. You're equating the blood and other parts of your body to the energy source. I would personally equate it to more of the transfer network for that energy. It's sort of like plugging a 110 plug into a 220 outlet. (Trust me, not a fun experience.) The wires will still transfer the electricity to the device you plugged in, but they will also be damaged by using more than they can stand. (And possibly blow up or catch on fire. Like I said, not a fun experience.)
In this case, imagine that your... let's just call it a soul... is your energy source. It has an amount of energy that it can put out, but some small amount of that energy is held back as your "life force" because it powers your body and you need it to live. The rest is what you know as ki or power level. Training increases both. (Not that that's important at the moment.)
So when you fire off a special technique that uses "life force", you send the extra energy through your body... which harms the weaker passageways that your body's energy normally flows through. For your average person, whose pathways are already very small and undeveloped, this kills them as their body literally can no longer get enough energy to live.
But for someone who has trained and developed, who is already practiced in manipulating energy in the form of ki, they have the extra can afford to damage a bit of their body for that little bit of extra power. They're still getting the energy they need to live through their other energy pathways, but their "life energy receptors" (for lack of a better term) are damaged, so they're receiving it at a reduced rate.
Now since it's your body, it'll regenerate. But it regenerates in an inferior state, just like a broken bone is never quite as strong after it heals as it was before.
So that's why your life is effected, at least that's how I've always viewed it. If it was just "burning" the body for extra energy, then the body would heal no different that if they got cut and lost a lot of blood.
And... I was going to say something else, but seeing as it's nearly 4:30, I've lost my train of thought. Something something blood is such a small part of your body that you wouldn't be able to get much out of it or something. I honestly don't remember.
So I'll just jump into the other part of what you said that I think wasn't well thought out. First, let's forget everything I said above and focus only on your model of power energy stuff thing. You say that we can burn 10% of our blood for that extra... to use your percentages and our base power level... 2,625,000 PL in base form, 10.5 million at the highest strenght we have. (Golden Oozaru.) Sure, that would make for a much, much more powerful attack... but at the same time, would we be able to fight? You're basically taking the highest of high ends of when people have blood withdrawn, and in the middle of a fight to boot. Even the people who say that taking that much blood has no side effects still will tell you to not do any strenuous exercise for five hours and to rest and take in plenty of food and water afterwards... and that's with the blood going out in a steady flow. Having it all disappear would be disastrous, what with the 10% instant compression of your blood vessels... but even having us just lose the blood cells and leaving the plasma would still be
inducing mild anemia for a rather insignificant power boost. All in all, for such a tiny boost it wouldn't be worth it when the possible (and yes, this is only possible) side effects include weakness, fatigue, dizziness and the like. And it doesn't just return diminishing returns, but the more we raise our power level to increase our ability to fight, the more we utterly destroy our ability to fight by
inducing anemia.
In other words, you'd have to blow five to six of these
in your most powerful form to equal the power level that Jaffur had
years ago before his sealing... and at that time you'd be missing about 55% of your blood and, to quote Medical Daily "The final classification of hemorrhaging, Class 4, occurs when a person loses over 40 percent of their blood volume. A hemorrhage so severe requires immediate and major resuscitative help, or else the strain on the body's circulatory system will be too great to survive. The heart will no longer be able to maintain blood pressure and circulation, Alton said, so organs will fail and the patient will slip into a comatose state preceding death."
Or, to put it even plainer: this is the Dragon Ball Universe. As far as I can recall, there has NEVER been a battle that was won because the victor was just a
tiny bit more powerful than the opponent. In battles at power levels like these, if the difference of power is that small there is no difference than if their power levels were the same. It all comes down to skill, technique, and sheer willpower and grit then. Power levels only matter when there is such a huge gap that it MUST be overcome by teamwork, development into a new transformative form, or author fiat.
And again, this is only from what I saw in Dragon Ball, DBZ and GT. Haven't seen Super yet, so anything there that contradicts this... well, guess I'll have to re-think this whole thing.