The cause of the failure is not upgrading the gears, it is driving the car with upgraded gears at twice the speed limit.
You can't just slap on an aftermarket performance part (in this case gears) and push the car's limits without accepting that you might require other prerequisite upgrades
And my stated "5% of owners" is probably a gross exaggeration
Better engineered driveshafts are available. If you think Ford / Ford Racing should offer an aftermarket driveshaft, or even as a factory option, that makes perfect sense, but to put in a performance driveshaft as a stock part and bump the price of the V6 is a bad business move.
A stronger driveshaft would fall under the category of a ''would be nice feature", it does not fall under the "must have". Ford set the stock speed limiter to 115 or so for a reason so people don't abuse the driveshaft. For the 2011 v6 mustang Ford only offers a tune up to the 3.31 gear ratio last time I heard. So going with 3.73 like I have or 4.10 require a special non-dealership tune which they don't necessarily encourage or support. If you modify a car you have to look at all aspects and make a decision to modify weak points that can fail due to the modifications.