Totally agree that stimulus spending has diminishing returns. Especially in leverage situations.
But I disagree that fiscal/stimulus policy is shot (and for the record, I don't think it would be effective right now anyway). For reference, the S&P 500 companies uses of cash since 2007 on stock buybacks went like this: 33%, 25%, 23%, 21%, 28%, 23%, 26%, 27%, 24%, 25%, 24%, 28%, 28%. So in '18 and '19 they did a little bit more as a percentage of cash but not drastically different. Dividend history was similar, fyi. Dollar amounts were higher but that's because they had more cash. Regardless, buybacks and dividends were there before and didn't increase drastically due to tax stimulus. Capex percentages were similar in percentage history too, just more on a dollar basis.
So, respectfully, your analysis is wrong and you should feel bad;). There is plenty left in the tank, should the Fed and government feel the need to. But this sell off has fuck all to do with that and more about unknown impact to economic activity of everyone on the planet sitting at home in fear of this virus.