IGY, PRICE IS NOT THE SAME THING AS VALUE.
If everything really was overvalued, nobody rational would be in the market. That is, people still perceive value in even highly-priced stocks.
"Value" can take many forms--pride of ownership, prospect of return, competitor blocking, filling in missing parts of a whole, etc. All these things, and more, apply to stocks and bonds--and the "more" can often be totally irrational, motivated by emotional need rather than considered reflection.
Also, market indexes do not wholly reflect economic activity; that is, economic downturn is not synonymous with market crash or correction. "People" learn different things from the two different types of experiences.
With the possibility of widespread market participation in the US via 401k's, etc., IMO it was hoped that everyone would feel as though they had a stake in market, and by extension, capital, and even maybe economic, performance. Unlike in a business that offers stock as compensation, however, the connection between the individual and the market/capital/economic performance becomes vastly more nebulous and remote. As far as "markets" go, many rationally understand that it is a game played with a marked deck, but nonetheless remain in, because of emotions like hope, fear, greed, etc.
And those things are your friends if you are trying to make money in the markets. If you are just trying to preserve wealth or go with the flow, the markets can be useful (although there may be better vehicles). Consider coach and his NFLX. Have the assets of NFLX appreciated as much as the stock price while he has been holding it? No. Projecting future performance, of both the business and the stock, is where the emotions come into play. Coach has profited handsomely from both his analysis and emotions, and from the analysis and emotions of others, same as he does with his short positions. He is in the market to MAKE money, and apparently, he does. I know other people who do the same.
The only answer you can have to the fact that he makes money on NFLX is an argument based on time-frame, or data windowing. He could realize gains today. For him, NFLX is NEVER "over-valued" while he is holding it, and lots of people hold NFLX, AMZN, AAPL, etc. As long as there is widespread holding of these stocks, value perceptions of people like you, who do not hold them, are at best only half the story--and probably less than half the story, because those who DO NOT own the stocks lack the control held by those who DO own the stocks.
You are arguing from a personal perspective. There is no absolute reference in this story, unless you wish to consider asset liquidation value of each business, which is an absurdity unless it does nothing other than hoard physical commodities.
This is why the boomers retiring and shifting out of equities may be troublesome going forward. They are retiring, and rebalancing, in droves. The overall social calculus is IMO likely to shift in your direction, toward a perception of over-valuation.
But for now, plenty of people are long on these stocks, and they have carried the day--and legitimately, at that, no matter what the current price.