Very crudely, a 20% drop in revenue for Alphabet would almost wipe out its profits.

Alphabet's revenue is a mix of hardware, online store, web services, and ad sales. With a global slowdown -20% is probably in the ballpark, although it may take a year for the effects to accumulate and ripple through.

Roughly half of GOOGL's opex is R&D, so I would expect some cuts in that over the next year or two. Maybe also in the more exposed parts of sales and marketing.

If you have sufficient reserves to last it out, then a year or two with no margins or negative margins should not mean a huge decrease in company valuation. If you lose 20% of your next 10 year profit because you lose 2 of 10 years, then that that justifies a 20% drop in share price but not a 50% collapse.

if you believe that, then I'll trade you 20% of my expected 10 year income for 20% of my current discounted net worth (which is theoretically what a company's market cap should be, a discounted claim on future distributions).

revenue today (or in the shorter term) is always more valuable than revenue 10 years out. put it a different way, what if there's another "act of god" in a few years? the probability of an extreme event occurring within a discrete timeframe increases as your timeframe gets larger, thus your certainty in revenue projections should decrease as you project further out.

Sure, but having good margins in the first place gives you more room - you might call it, "margin" - before any of that results in you actually losing any money.

Revenue 100, Margin 20, Profit 20

Revenue 0, Margin 20, Profit 0

Margin is only helpful if you have revenue, this will hit Google and other ad based companies hard (though google probably has the cash to weather it), Amazon will be fine and may even benefit.

I saw a sudden drop of about 25% in ad revenue across the properties I'm privy to two weeks ago, but it seems to be pretty flat since then.