If it became too difficult for them "Charge Facebook $100mm" they would replace support with a lower quantity web app and blame Apple.
The reality is that Apple don't have as much interest in the web anymore, they have achieved their goal with web standards (removing enough of a dependency on Flash so they didn't have to support it). They have no need to make the web a successful app platform, so why waste effort on it.
For Google it's part of their core business, look at Angular, Closure tools, Dart, GWT etc...
No, it's not.
https://investor.google.com/financial/tables.html
Total Advertising Revenues: $59,056,000,000
Total Other Revenues: $6,945,000,000
All of this happens though the web. Native apps and things like android are just a way to drive people back to the web.
Now consider Google have chromebooks, where those apps must run on the web.
This depends entirely on how you measure scale. If you do it just by number of apps/sites, sure, the vast majority of web URLs don't have a matching iOS app.
However, if you scale each app by the total amount of time users are on it, you'll find that most of humanity is spending their time on a very heavily weighted small slice of the Internet.
From that breakdown, the fraction of human-hours that is covered by sites that also have native iOS apps is much larger.
If the next version of iOS didn't even have a browser, you would be astonished at how little that impacted most users. As long as they can still run Facebook, Instagram, Threes, etc., they'd be fine.
If time spent in an application is your criterion, true.
But that way you hugely underestimate the importance of, for instance, that daily (or even weekly) Google search the average user performs in their browser. It takes little time compared to your average social feed, but definitely impacts their lives. Just one quick example: a local business' phone number lookup.
I don't think it's useful to downplay their influence.