As a humble passenger, but someone who has often had to delegate technical work to others, I confess I'm surprised that all blame landed on the mechanic.
Surely he wasn't just handed the task, and the aircraft passed fit to fly because he said he'd done it? Doesn't anyone check repairs? Would no-one have seen that the finished job didn't match what the manufacturer specified, or the airline's practices stipulated? Perhaps they wouldn't have been able to identify that it was titanium rather than stainless steel, but surely they would have seen the problem with the holes? And if they saw it, and passed it on the basis that it was 'good enough', doesn't that propagate responsibility to line management? Was the supervisor cleared because it would not have been possible for him to check the repair, once done? Is this kind of maintenance autonomy common within the industry?
I also can't judge how rare it is for such bits to fall off planes. I presume that wear strips, by definition, get worn. Is it common for them to fall off? Or do they tend to stay attached if they're made of the right material, and correctly fitted with the right number of bolts through the right number of holes in the right position?