All of it is highly situational, and it depends on a bunch of adventure specifics as well as how you define durability and how you define decent damage input. That said the numbers generally in my opinion are not what drives this as much as the mechanics and situational factors. Here are a few reasons I said what I did.
1. Fighter has less total hit points to start the day (i.e hps vs hps+temp hit points) and in terms of not going to 0 having those hit points on the front is better than having them available on a bonus action.
2. Second Wind can't give more than the maximum hit points and therefore can't be used as efficiently as Armor of Agathys. The average rolled on the dice for second Wind is 6.5 hit points vs 5 for Armor of Agathys. But the average hit points given for second Wind is lower than 6.5. This is because for the average (mean) to be 6.5, the maximum (11) needs to be possible. A Fighter with say a 16 constitution is going to have 13 hit points. So the average (mean) he rolls on second wind is 6.5, but ONLY if he waits until he has 1 or 2 hit points to use it. Using second wind with more than 2 hit point in this example puts the fighter in a situation where the average hit points gained is lower than 6.5.
3. The Warlock's AC is lower but it is competitive when you consider Blade Ward and Shield. One use of Shield is typically better than one use of 2nd Wind, or for that matter one use of Armor of Agathys and if you assume 2 short rests that gives him 3 uses of shield a day, plus one use of AOA at wake up, plus the ability to cast AOA as an alternative if the situation warrants.
4. Finally the largest reason is to make a high AC fighter you need to either go into melee or accept short range and lower damage. This in turn is a bigger factor in durability than anything else. IME characters die in level 1 and level 2 more than all other levels combined and melee characters die significantly more often than non-melee characters. Not having a shield enables the Warlock to do better damage from Range, which translates into more durability. A fighter could do better damage-wise with a Longbow or Crossbow but when considering spells, they will accept a lower effective AC to do that.
Like I said though there are a lot of variables, but this is how I see it. I could easily be wrong though and certainly would be wrong in certain campaigns, situations or tables.![]()
I wouldn't wait until I was at 1 or 2 hp to use second wind. As soon as I'm below half hit points the averages work out and I'm not creating unnecessary risk. That's anywhere from 1 to 6 hit points.
We also cannot omit weapon masteries creating disadvantage on attacks. If I go with the sword and shield style mentioned then make 12 attacks in the day that's ~8 incoming attacks at disadvantage. Disadvantage against AC 19 in 1st level encounters is significant.
Goblins and kobold minions, for example, are +4 to hit.
The warlock example has a 20 AC for 1 round and that's it if we're arguing armor of agathys; or the warlock has 20 AC for 1 round per short rest if we're arguing shield instead. It can't be both ways because the warlock only has 1 pact slot per short rest. The fighter has 19 AC all the time and inflicts Sap once each of his turns, and possibly on a reaction.
That changes the chance to take a crit from 1 in 20 to 1 in 400 from those attacks on top of leveraging that high AC.
The problem with the warlock is the thp are pretty much gone in one hit and that AC gone after that one round.
EDIT: I'll also point out that the fighter would start with 3 more hp than the warlock as a good chunk of that thp too.