Given how armor works in practice in F1/F2, I am extremely skeptical that the DR/DT system was designed to "combat" picking the armor with the highest DR/DT.
Exactly. DT/DR was there to force you to make decisions about what
weapons and ammo you use against various types of opponents, not what armor you wore. Each iteration of Power Armor was objectively better across the board than all previous armors, with only Tesla Armor being even close to competitive. Heavier armors even gave you better Evade/Armor Class than lighter armors did, so there wasn't even a trade off as far as evasion vs. sheer toughness went. In Fallout 1&2 best practices meant wearing the heaviest gear you could strap on while still having an inventory-- And Power Armor boosted your strength, so there went THAT concern.
To be honest, I'm glad they're ditching Damage Resistance since it is less intuitive than Damage Threshold even if in theory a DR type system scales better. Besides, trying to make armor piercing ammo that neutralized damage resistance while having a lower base damage just didn't work very well once you factored in that such ammunition did nothing vs. Damage Threshold. Even a -100% damage resistance modifier on your ammo will be useless if it comes with a damage modifier so low you that you almost never beat their Damage Threshold, which is why by mid-game putting AP ammo in an full auto weapon was usually a useless gesture. For that many different modifiers to really work out you'd need to make a game that is a lot mathier than most players (and hell, even devs) want to deal with.