I do not believe tension or "hardness" has much if anything to do with how long the string will last. I believe the tension is more a function of the materials modulus of elasticity and how much tension (force actually) is required to tune the string to pitch. just engineer talk
Like I said earlier, it is dirt, sweat and skin oil that kill the basses and the trebels just get scratchy (maybe over stretched if they are REALLY old. A simple test is to pull the harminic at the 12th fet and compare to the fretted note at the 12th fret. If there is a difference in pitch, the string is not vibrating evenly along the entire length due to accumulated dirt or eneven stretching. I have heard that some people double the life of the basses by boiling them for a minute or two in mild soapy water, but it seems a bit of a bother. If you try that, I would suggest taking them off of the guitar first.