I'd also argue that no reasonable amount of defensive layers will completely prevent you from being oneshot
If that is your only actual goal, theres plenty of ways to achieve that. The easiest one would be Transcendence.
I do agree that not a whole lot of players reach red-maps in the first place, but then I don't really consider anyone who hasnt even finished the Labyrinth to be an actual "player" of the game. Once you remove the lower echelon of players who maybe have 20-50hrs in the game and barely finished Acts1-10, PoE is one of those games with a fairly large percentage of players that consider themselves "experts" even if they clearly aren't.
But despite the amount of people spending considerable amounts of time in PoB being much higher than in other games where similar min-maxing is possible, most of these min-maxers in PoE are still whiny manchildren who cant build a good character or not die every second map for shit. Even if they copied Dans latest HC character, theyd probably still rack up dozens of deaths, and not because the game is unfair, but because they simply suck at the game. And sucking at the game is fine for 99% of it, but there are times where you actually need to know how to deal with certain bosses or combination of mapmods/affixes, and its not due to the tools being given to the players being inadequate, but the players simply aren't using them. They look at a timeless keystone and if it doesnt give any damage its generally considered "useless", same as flasks with big defensive benefits.
Now the question is, whether this is really a problem of the game not offering solutions to the "oneshot" issue, or if its an issue of it being underused, or not accessible enough, or it being an obscure unexplained combination of mechanics that no one understands unless they played the game for 5000+ hours. I would argue that its just people wanting to have more DPS in PoB, and neglecting defenses, instead of coming up with innovative ways to make their character more tanky. The main issue that PoE has, is that for the largest share of the game you do not need multiple layers of defensives, and increasing your DPS will provide more safety than any extra EHP would. But if you keep dying every second map to that one remaining percent, is it really worth it? Maybe slowing down the DPS progression will shift these ratios a bit, and being able to out-dps everything is not as feasible of a thing anymore and thus people will educate themselves a bit more on how to build defensive characters, not just chase the biggest PoB DPS number.