A ton of online services, customer support services, government services, airlines, etc. etc. use e-mail but don't use gmail to send it.
They can't afford to let 20% of that go to spam without a backlash. And if they kept doing it despite the backlash, who knows what would have happened to gmail.
Lose a few thousand dollars or an entire vacation by google regularly blackholing messages from important and expensive services like airlines, when they send you email telling you your flight departure was changed, and you'll be looking for a better email service in no time.
Now imagine this happens to 1/5 of the customers of said airline, just because of google's 1/5 non-google mail go to spam policy. It would be a scandal.
They can't afford to let 20% of that go to spam without a backlash. And if they kept doing it despite the backlash, who knows what would have happened to gmail.
Lose a few thousand dollars or an entire vacation by google regularly blackholing messages from important and expensive services like airlines, when they send you email telling you your flight departure was changed, and you'll be looking for a better email service in no time.
Now imagine this happens to 1/5 of the customers of said airline, just because of google's 1/5 non-google mail go to spam policy. It would be a scandal.