So I did some comparisons. Earlier I said that AC values in 1e were overall higher than their counterparts in 5e, and Hussar laughed it off. So I pulled some of the most common monsters and sample size (did not cherry pick). Every time there is a blue highlight, that means the 1e monster has a higher AC than it's 5e counterpart (naturally converting the descending AC in 1e to ascending).
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Also, I ran comparisons for the core classes re: hit points, bonuses to hit, and total available hit points in a 24 hour period (1 day). I've often heard the "well, but 5e increased monster damage a lot". Looking at the above, if you compare monster damage and to hit bonuses by monster hit dice (you can't really compare monster vs same monster name, because some monsters like orcs were bumped up. I.e., in 1e, an orc was a pretty close to a 1st level character, while in 5e, it's equivalent to a 2nd level character). So monster hit dice is a better comparison. When you compare by hit dice, there isn't any real difference in 5e monsters being able to have a higher to hit bonus. And damage is only slightly higher.
But higher damage doesn't equal more danger more lethality because 5e PCs have a LOT more hit points available to them in any given adventuring day (not just with higher hit dice, but with having hit dice to heal up every short rest, and getting all base hp back after a long rest).
Note: For simplicity, I did not count any ability bonuses to hit or for extra HP. If I did, 5e would have an additional benefit because you increase your core abilities in 5e and you do not in 1e, and every class can benefit from high CON when in 1e, non fighters were capped at a +2 bonus.
So what does that look like?
To hit bonuses: 5e PCs gain a better bonus to hit at lower levels (everyone starts with a +2 prof bonus, and 1e doesn't have this). At mid levels, the fighter begins to outpace the 5e bonus to hit counterpart. Clerics also being to outpace at mid to higher levels. Magic users are always way behind in their basic bonus to hit. High level fighters way outpace their 5e counterparts in core bonus to hit (doubling up even). When you add ability modifiers, this evens out considerably (as without magic items, most 1e fighters with a 16 STR will only have a +1 to hit with melee weapons, while a high level 5e fighter will have a 20 STR with a +5 bonus.
Hit points are significantly different. again, not counting ability modifiers (which make 5e version even better, not just for higher ability scores that can grow, but because you stopped getting CON bonus for HP after level 9 in 1e), the 5e fighter has about 20% more base hp than the 1e fighter. The 5e wizard has double the hit points of a 1e MU. For comparison, at 10th level:
1e fighter: 53. 5e fighter: 64
1e thief: 33, 5e thief: 53
1e MU: 24, 5e MU: 42
1e Cleric: 46, 5e cleric: 53
At 20th level, the 1e fighter will have 83 hp and the 5e version will have 124. The MU has 34 hp and the 5e one has 82.
But wait, there's more!
When you factor hit dice healing (1 HD per level), and gaining full hp after a long rest, while a 20th level fighter in 1e has 84 total hit points available in an adventuring day (83 plus the one for resting overnight), the 5e fighter has 348 total hit points available. Core 124, plus 20d10 hit dice, plus the core again that can get healed completely after a long rest (a heck of a lot more if you factor in CON bonuses)
So, even though 5e monsters can do a bit more damage per hit dice, the actual net affect is less because non-magical healing and total available hit point resources are so much higher. Eg. monster A in 1e might average 10 points of damage a round, and the 5e hit dice equivilante might do 15. That 10 points is more devastating to the 25 hp MU than 15 points is to the 42 hp MU.
Once again, I am not arguing that 1e is better in any way, shape or form. And I'm not arguing that a DM just can't boost 5e monsters. (I hope this means people will stop making those arguments) But it's clear that looking at how each edition was designed, even if you totally ignore save or die, or level drain, or long term conditions, and only look at the core damage, 1e is still significantly more lethal/dangerous to PCs. To argue otherwise just seems like a silly position. All of the evidence points otherwise.