It seems to me that, since HTHS is a measure of viscosity, one would want it to be within a reasonable range, not too high, not too low.
HTHS is a virtual dynamic viscosity at 150°C and a shear rate of 20 m/sec. It was "invented" due to the fact at the beginning of the 80's engines failed with oil of an HTHS < 2,0.
During a SAE congress in the 80's, all members agreed, that the available data showed that an HTHS > 2,5 is fair enough for every engine on the market in those days.
High HTHS values give you a fuel penalty. Depending on your engine, you will experience a higher fuel consumption of 2% if you use an oil with HTHS of 5 instead of 2,9.
Either your engine is durable, or it is not. If it is durable, high HTHS values don't give you a "safety margin". Thats especially true for your normal car, and that's also true for Diesel engines.