I don't agree it is a house of cards like the 2008 RE bubble or the 2000 dotcom bubble, not for a good while anyway. Money supply increases of a reserve currency as powerful as the dollar simply becomes a fee charged to everyone across the world who hold dollars.
Moreover, when we look beneath the indexes, we see the stocks of travel, entertainment, energy, restaurants, small business have remained at deep recession/depression level prices. Overall, the indexes are holding up because rapidly growing companies are winning faster than expected due to the crisis. Both already giant companies and the best upstarts have soared since March.
Nevertheless, IMO, over the next 15 years or so, there will be massively more wealth created than at anytime in history for that time span.
It is the worst of times and the best of times.
Many will suffer from displacement, unemployment, and increasing poverty.
But if you invest on the right side of history or on the right side of the technological evolution landscape, it will be the best of times for you, and give you increased means to help others.
Over the next 10-15 years, a good guess is that most of the current S&P 500 index will be replaced by new companies, companies that are shifting away from the industrial age to the digital age.
There has never, ever been more opportunity for the educated, the innovative, and the motivated.
Peace.