The thing is, every time there has been a bubble, one of the major arguments always presented is that well, this time it's different so the traditional measures don't apply.
In 1999 it was increased worker productivity (due to IT advances) that supposedly supported the outsized P/Es. That turned out to just be a smokescreen, of course.
It might even be argued that an excellent indicator of being in a bubble is the existence of arguments about why it's not a bubble even though it looks like one. That means you're at a stage where even the bullish market forces admit that traditional metrics do not support the increased valuations.
But if we get the wave of mortgage, debt, and business defaults that the Fed and Treasury are trying to prevent, that angel cash bubble may feel the pinch too. They're still fighting it.