Fanaelialae
Legend
Well, it isn't easy as pie, but I don't think it is as bad as all that. For one thing, you can do it in more ways than merely changing the hit point/expected damage ratio directly:
RC weapon mastery style, a bit of a genre emulation method - throw in some parries and other such active defenses that are practically non-existent at low levels, but get better as you go.
Indie metagaming - those "fate" points start at zero but go up steadily as you level.
Magic will do it, kind of a mid to high fantasy trope - provide a few extra spell options at higher levels that can really cut out the danger for a time.
The old standby, magic items - get more generous with damage mitigation items.
Naturally, all of those can also be set on a more steady scale, whether you want a lot of lethalness or not.
The more consistent lethalness is in the base, the easier it is to make all those flexible. Maybe some people want "moderate lethal" at a given point, then you get the right spells to hold it off, and when you run out of those spells on a particular high level adventure, you are just as much at risk as you were at 1st level.
This seems to me like it would be vastly more complex (and therefore difficult to get "just right") than just a single dial. The more dials you throw in the more you muddy the water.
As for making lethality dependent on a spell (or weapon specialization), well... it doesn't strike me as a very good way to approach lethality. In the case of the spell, you're basically saying "memorize this spell or die". That's terrible game design. A game like D&D is about making choices. You can't make choices if you're dead. Giving the casters the choice of "spell or death" marginalizes their ability to make those valid choices, and is therefore bad design.
Some of your other methods might work, but are either require a particular style (every PC must have heavy fortification armor if you want to run a low lethality campaign) or are just plain clunky (you're impossible to kill until you run out of fate points, and then you die).
HP have always handled lethality, and have done so respectably. I don't know why you'd want to use anything else as your primary lethality dial, aside from perhaps damage (which really just approaches the equation from the other side).