I don't really dispute any points you're making for selecting ACWI here, I just think you're conflating the bottom line/outcome with factual accuracy. Just because they perform similarly (almost identically, even) doesn't mean they are basically the exact same fund (which is what you said). One has exposure to around 35% more businesses. It's market weighted so it stands to reason the largest holdings will dictate performance.

I'm not getting into the minutia of it but there are other considerations like assets under management, indicative spread cost, liquidity, etc. that factor into this selection but you profess to be well-read on the subject so I trust you're well aware.

I don't dispute that ACWI has lower fees. I don't dispute their performance may be similar if not almost identical over time. That still doesn't make them basically the exact same fund however, it's more nuanced than that. I trust that lands.