It's hard to control conditions for a Marathon, but what's to stop some future Olympic host from intentionally creating the fastest pool possible that wouldn't be objected to, just so they could watch "their" Olympics break all the records?
That is exactly what happens almost every time... Just not this time
Don’t we want Olympic hosts to create optimal conditions for athletes to demonstrate their skills?
We want Olympic host cities not to go into billions of debt that they'll never make back, as that will make it hard to find good hosts. Paris tried to be prudent tinancially.