Any long-distance athlete would be intimately familiar with this trade off. At Ironman races you always see a non-trivial fraction end up with debilitating cram

Any long-distance athlete would be intimately familiar with this trade off. At Ironman races you always see a non-trivial fraction end up with debilitating cramps, confusion, nausea + vomiting, or ultimately in the medical tent from low sodium or low glucose. That said, I think "drink water" is still good blanket advice on average. Most athletes I know still drink far more water than the average person through the day, in addition to all the sport-specific fuelling.