My guess is that the answer to that is cheap debt from the fed keeping interest rates so low for so long. The stocks had value beyond what you'd expect from P/E ratiosand assets alone because there was added future value in the form of anticipated stock buy backs.
That really depends on your timelines and risk tolerance assessment including externalities.
To use poker as an analogy: if you aren't sufficiently bankrolled, the correct play (most positive EV) can involve an unacceptably high risk of ruin. ....and BTW are you also factoring in the probability of fraud or a government seizure in your calculations?
'flat' only if you see through the shenanigans; otherwise, EPS (earnings per share) have increased, by reducing the outstanding shares.
When interest rates are low, firms can finance to buy back. Not only that, investors shift their investments to stocks, instead of bonds. Two forces are at work to pump stocks, both forces are products of low interest rates.