Long Term effects of saturating isolation txfr
When the transformers primary is connected to source (mains power supply), then the flux in the transformer is nominal, even if the secondary is left open. The xfrmr can't be saturated by means of overloading it, therefore your theory doesn't stand. If the transormer is overloaded 15VA instead of 10VA, then it will overheat and the coils will melt down.