Well, maybe this is the part I don't understand. I thought the burn is from lactic acid, and lactic acid, if not re-absorbed, will interfere with muscle operation, i.e. failure/cramps. So, when the burn starts, and I just keep pushing, or don't leave long enough recovery between efforts, something is going to happen sooner or later, correct?

For regular rides, I have no issues going 50-100 miles in 95 degree heat, I just manage the (effort)x(time) equation such that I don't overdo it.

If hydration/electrolytes were the main issue, wouldn't the threshold be more variable, dependent on initial hydration level, temperature, etc.? And why can I recover strongly, I thought once dehydration/electrolyte deficiency sets in, there is no way to recover on the bike?