>>529874039
>>529832827
>>529812197
The apples-to-apples example I think of is the Soviet tanks

>the T-72 is a mass-produced workhorse that's still following late WW2 doctrine
>the T-80 is a much more expensive and advanced tank with cutting edge features like composite armor, night vision and computerized targeting
>the T-90 is a T-72 hull with certain electronic and mechanical upgrades learned from the T-80 integrated into it, resulting in a tank that's still not as good as a T-80 but is much more capable than a T-72, because you already had 100,000 functional T-72 hulls laying around

Within the Fallout canon, the US absolutely could not and would not afford to just dispose of its vast stocks of T45s midwar just because the T51 was objectively superior. Every suit of power armor is valuable; hell the majority of soldiers were still stuck with combat armor

PS, this bit is all headcanon and excusing Bethesda mistakes but I don't hate the idea that T51s would be all over the West Coast because they'd be on their way to Alaska or even across the Pacific, while the T45s found around the Pentagon would essentially be reserve and National Guard units, like how in real life the National Guard kept getting Garand rifles and Patton tanks decades after they were obsolete. FNV was even supposed to play with this, giving T45s to lower ranking Brotherhood Knights and T51s to higher ranking ones, but it isn't shown ingame because they literally programmed the spawn tables wrong lolobsidian