Only big brand advertisers (like Clorox/PG/etc) evaluate ad spend on the impression/click level. This is not the norm (unless you want to lose your shirt to bots/fraud). We have made it a long way from the blunt-instrument advertising of past years. Full disclosure: there may indeed be a 'changing-of-the-guard' happening with regard to brand advertisers waking up to the available tech, but that is a different story..
Overall, this is a non-story. Many corporations were slapped with a massive bill after the passage of Trump's tax cuts. Not just Google. So with that liability, of course PROFIT dropped. But the REVENUE is what you want to look at for these 'bubble' discussions (and that is strong), so nothing to see here...