I am not certain how a 20-50 oil provides more protection for the engine in hot climates. These oils are multi-viscosity, which means a 5-50 oil has the same thickness at high temperature as a 20-50. It just flows easier when cold.
With conventional oils, such a wide spread of viscosity range is accomplished with additives that break down. Thus, using a conventional oil, a narrow range is "better" provided climate conditions allow, because it needs less viscosity index improver due to the narrower range. But synthetics do not require much, if any, viscosity improvers, so there doesn't seem to be much advantage to running a 20-50 in hot climates.
If the engine requires the lower viscosity at cold for lubrication due to tight tolerances (which I do not know is the case for the 2zz engine, though many have indicated it as a possibility) a 20-50 would be a detriment.
As far as manufacturers just specifying "thin" oils for gas mileage, with no regard to longevity - most manufacturers have 7/70,000 mile engine warranties (i.e. far longer than they offered 15 years ago) so I don't know that they don't care about engine longevity. Of course Lotus is only 3/36,000, but I'm sure Lotus doesn't have to worry about CAFE too much...
DLY