In fact, some economists continued to argue after 2000 that “bubble” is a meaningless category. Uh oh. "Stocks have reached a permanently high plateau" - Irving Fisher, economist, early October 1929.
There is something different this time. People en masse have started using the S&P 500 and other index funds as a long term store of value. Large amounts of money buy these stocks every pay period with zero regard to performance or current market conditions. As far as I'm aware, this has never happened before. We don't know how this experiment will change the market long term or whether it will eventually collapse.
There always is. And then, abruptly, there isn’t anymore.