S0rcererV1ct0r it's hard to give you an accurate and extensive reply because you say so many things that don't make any sense (about 5E obviously. I know I can be a giant autist, but I don't want to attack you personally).
Wrong. A lv 1 warlock can with small chance OHK another lv 1 warlock. But a lv 4 warlock, can't OHK a lv 4 warlock even maximizing the rolls. HP growth is far greater than damage growth.
While this can be a true statement, it's completely irrelevant because in 5E monsters and encounters work differently than previous editions. In 3.5 you could build an enemy of an appropriate CR with character classes, but in 5E this is no longer true. A level 4 Warlock is not an appropriate challenge for another level 4 Warlock, that's why the lethality in a warlock vs. warlock duel can be (in general) fairly low, but this says nothing about the general lethality of the whole game. It's exactly like saying that 3.x is a low lethality game because a goblin can't one-shot a level 15 character; yeah, no shit, it's not an appropriate challenge for that character.
Without even addressing the specifics of how CR works, it's easy to see how lethal this game actually is (at least before the highest levels):
1 - a level 1 Warlock should have around 9 or 10 HP, 11 if built with a race that boosts Constitution. Now let's see the potential damage output of a couple of CR 1 monsters: the
giant toad can dish out 2d10 + 2 damage in a single round (average: 13), the
tiger can deal 1d10 + 1d8 + 6 (average: 16), the bugbear deals 2d8 + 2d6 + 2 damage in the first round of combat (average: 18). They can all take down the character in a single round.
2 - a level 4 Warlock should have around 24, 27, or 30 HP. Now let's see the potential damage output of a couple of CR 4 monsters: an
ettin can deal 4d8 + 10 in a single round (average: 28), an
elephant can dish out 3d8 + 3d10 + 12 damage in a single round (average: 42 damage), a
wereboar can deal 6d6 + 6 (average: 27 damage). So, on average, only the sturdiest Warlock possible will be able to survive a single round of combat (and we are talking about a Dwarf Warlock that put his highest score in Constitution).