Advertisers don't always insist on seeing evidence of genuine ROI but online advertising is more measurable than most other forms of ads and when the ROI slips a lot of ads customers notice and act accordingly.
The number of people who don't see their bottom line growing, but are spending on ads, getting views is not reported anywhere.
Would also be interesting to see if ad budgets of the largest spenders are increasing with time once market capture is complete.
It’s actually shocking if you think about products from your childhood which are no longer advertised and have completely dropped out of public consciousness.