Well
1. Whats the point of power armor in DT if leather armor + chems and perks can match it as well? Its possible to achieve the same thing in NV.
2. The DR cap in Fallout 3 is 85%. The single suit of T-51b power armor in the game will only get you to 60, and even with the 1 rank of toughness(which adds + 10), and Barkskin(which adds +5) you will still wont reach the DR cap. And that is disregarding the fact no merchant can repair the suit to 100%, meaning you can never have it at max DR. The only way to reach max DR in Fallout 3 without power armor is to just spam Med-X(+25) all the time(and that requires massive hording, spamming the use of it, and seeking chem addition cures if you haven't taken the perk), or by exploiting Nerd Rage(+50). both of which are playstyles that require a lot of micromanaging.
3. Your point about Geckos only shows another flaw in your reasoning. If PA is supposed to be so great, how are Geckos damaging your at all? Even with critical hits, a Gecko just has small teeth, and some claws, and should be doing no damage to your PA. You went from "PA should be able to deflect all this damage because logic!" to "PA should allow you to be damaged by things it logically shouldn't if game magic is involved"
All you have really shown is that Fallout 1/2 had poor internal consistency(mechanics wise), and that Fallout 3 and later games needed to limit how much of a chem you could take in a certain amount of time to prevent things like Med-X spam, as well as introducing a timer on things like Nerd Rage that only let it activate once every X seconds.
1: Uhm you can't reach power armour levels of DT with leather armour and such? You can get DR from chems, and a little more DT, but nowhere near enough. And who's to stop you from using those chems and perks WITH power armour?
Did you ever actually fucking use power armour to tank anything? Leather armour won't save your skin from a deathclaw, but at least power armour can allow you to take a couple more hits.
2: First off, I reached the cap of DR with the winterised power armour, and combat armour, via getting all the perks I could to buff my tankiness (Cyborg, pit warrior, etc).
So its obvious you never played the fucking game, just looked at the wiki.
Come back when you speak the truth.
3: Its a gecko which ate fucking radiation, or breathes fire.
Make this clear. BREATHING...FIRE.
And who's to say they don't get a lucky hit in? A good hit to the head can cause a concussion, or a good hit to the joints could cause pain and suffering.
Critical hits are an issue with burstfire guns in fallout 1-2, but single hit crits are fine.
In NV/3, medX isn't even that powerful, nor is nerd rage.
Because DR is lazy as shit, it resulted in the player tanking 90 hits from a deathclaw in FO3, while even the mightiest warriors can only take maybe 10 from deathclaws in FONV.
You can maybe tank some more if you take medX and other DR drugs, but that's excluding such buffs, which by default, FO3 lacks, due to it using DR as normal armour.