If you or your retirement is a shareholder in one of these companies, you'd rather they return $100 billion to you with boring buybacks or dividends than put it in an exciting AI investment that returns $1-$10 billion. Even though it didn't end up completely worthless, it's still a bad investment.
> If AI blows up in Microsoft or Google’s face they’re just going to write it off and spend a decade paying zero income tax
As a shareholder, you'd also rather they return $100 billion to you than put it in a bad investment that returns $0 -- even if that bad investment lets them write off $20 billion in taxes over the next 10 years you'd still be $80 billion better off.
> how much it’s actually going to matter. If AI blows up in Microsoft or Google’s face
An AI winter shouldn't be an existential threat to these companies -- but "the share price is 10% - 20% lower than the counterfactual where they didn't make these questionable investments" still represents a lot of lost wealth for shareholders -- billions (or trillions for the industry as a whole). A lot of that wealth is the hands of ordinary people, often through retirement investments in index funds.