Anybody can build a bridge that works - it takes an engineer to build a bridge that barely works.

So you're saying real engineers are all playing polybridge. Sounds about right.

The racing frames here don't break as a rule as OP suggests, if one does there was a manufacturing flaw or a crash/impact of some kind.

A few grams of weight does not have enough impact TT bike performance enough to make it worth shaving 100 grams if it risks reliability.