P/E ratios are probably more useful for comparing companies in the same industry at the same moment in time, rather than as a global rule of thumb.
Valuing a company needs to take risk free interest rates and future growth into account, so it's difficult to compare across vastly different companies and time periods.
Your response seems to imply that there are broad differences in potential for future growth (among other things) today relative to the dotcom era. Could you elaborate?
Alternatively, we could cherry pick a single company. How do you justify tesla's p/e relative to the other car companies? From where I'm standing it doesn't appear to make any sense.
P/E simply isn't a very good ratio other than just for a quick glance. Real analysts look at 5+ years future cash flows (not earnings), risk free interest, risk premium, cost of capital, and other factors.
Differences in any of those factors between companies or between time periods will determine why a company is valued the way it is. Interest rates are way higher now than a few years ago, and still lower than they were in the 90s. Some companies have cool business models that give them an edge, eg. Being able to generate lots of free cash, being able to invest cash at high returns, borrowing at lower rates, looking in customers, or making deals that will assure high revenue growth for the future.
I dont know much about TSLA but I don't think the market sees them as just a car company any more than Amazon was just a retailer.
Future cash flows are only predictions, you can't "look" at them. Anyone chasing future cash flows is chasing models of the future and st the whims of whatever the models did, and did not, as relevant factors.
Valuing a company needs to take risk free interest rates and future growth into account, so it's difficult to compare across vastly different companies and time periods.