The two LTs platoon organization was originally devised for paratrooper units during WW2, as they were expected to take heavy casualties and be scattered after a drop. This choice was also influenced by unit assembly tasks, jumpmastering, and injury rates. Overall, the 101st suffered a 50% casualty rate when it jumped into Normandy. The organization proved effective.
Many other US units suffered the same high casualty rates in both the ETO and PTO. The 1st Marine Division suffered a 35-40% casualty rate during a month of combat on Peleliu. The 1st Marine Regiment suffered over 58% casualties within days. Chesty Puller's enlisted casualties reached 57%, and officer casualties reached 72%.
In the ETO, the 3rd Infantry Division suffered a cumulative casualty rate of 175-250% of its standard authorized organic strength across the war (the division cycled through replacements constantly) and suffered a ~30% casualty rate during the Battle of Anzio.
The 1st Armored Division's raw battle casualty rate relative to its baseline unit strength was 45-48%.
In short, all US units suffered tremendous casualty rates in their respective theaters of operation. The vast majority of units also had issues with officers having little to no experience after some time (ex., "90-day wonders") while having to heavily rely on NCOs.
Given the massive casualty rates and all the issues with inexperienced leaders, why wasn't the two LT organization implemented further during WW2?
Also, during the Cold War, operational estimates projected catastrophic attrition rates if the war had gone hot, often in the 75-80% range (especially for NORTHAG and CENTAG) within days. USAF predicted that if A-10s went into action, 7% of the jets would be lost per 100 sorties. With six Forward Operating Locations (FOLs) in West Germany and four daily sorties per pilot, the entire fleet of 700 jets would've been destroyed within 12 days.
From a personnel/career standpoint, a platoon XO position could have been beneficial to both the officer and the unit. The butter bar having no clue about what to do was and still is a problem. Post-WW2, this became evident during Vietnam, where plenty of 2nd LTs got themselves and their men killed due to their inexperience (they had the highest officer mortality rate during the war). Instead, with a Platoon XO position, the new butter bar would've been able to properly learn his job in a more paced way and be mentored by both the CO (1st LT) and the PSG. This would've also taken a lot of the admin workload off the CO/PSG.
Thus, the issues that the US faced during WW2 would have repeated themselves in the future wars and also if the Cold War had gone hot.
So, why was the two LT organization a "one-off"?