What kills a BB....Easy answer the guy operating it. Many reason have already been pointed out. In some cases the block themselves are just inherently weak. We broke a street motor that had.090" thick cylinder walls. Had it been filled likely would have continued to work just fine until something else gave way. I am sure this will lite some butts a fire but MOST of the guys in here don't make bigpower and taking the usual precautions will likely have long lived BB engines provided they have enough cylinder wall and keep them out of detonating. Yes main webs do give way, this is nothing new and been an issue for a long time. The issue there is likely cap bounce, which will eventually destroy something. Again there are ways to address this as well. BTW even those fancy cross bolted main Mega blocks see cap bounce. Nothing lasts forever, all you can do is use due diligence when building an engine. Parts fail, the more they are stressed the more likely the lesser pieces are to fail