Earning ratios look wacked out because interest rates worldwide are lower than they've ever been for a number of reasons, like China and Saudi Arabia chasing yields and looking for return on literally trillions of dollars.

PE was lower historically because there wasn't global competition for returns, Europe was still fragmented and recovering from WW2, Japan was recovering from the bubble economy, China was becoming a faux market economy, etc.

I don't think it's relevant to compare PE ratios from 20 years ago to today, the world is a completely different place.

As for corporate debt, the dollar amount has doubled, but what about the interest rates ? If debt has doubled but interest rates have halved, nothing has changed in terms of debt service, you could argue it's actually a positive as companies can borrow twice as much money for the same cost.