PE ratio is one of the best tools to evaluate equities. A company that has positive earnings will have a positive PE ratio. But what is considered a reasonable PE ratio appears to be changing over time. I usually hear 10 as a very good PE ratio for company with stable earnings. If I was able to buy an entire company with a PE ratio of 10 and simply kept the earnings for myself and it didn't grow or shrink in earnings then after 10 years I would have my original investment back and all earnings after that time would be my gain.
However when I look at https://www.multpl.com/s-p-500-pe-ratio I see that since 1990 the average PE ratio has not dropped below 15 unlike the past. Looking at GDP growth I don't see the United States experiencing more growth in the last 30 years than before so that leads me to conclude that companies in the S&P 500 are experiencing the same earnings growth now as before 1990. So what has changed that justifies the increased price investors are paying for the same earnings and expected earnings growth?
One theory I have is that lifespans are increasing so people are simply willing to wait longer for gains to be realized. If my lifespan is 40 years and I have to wait 40 years for a gain that would be unacceptable but if my lifespan was 80 then a gain after 40 years is reasonable.
Are there any written materials or papers on this subject of increasing PE ratios since 1990?