How would that kill gmail? It's not like they're going to lose any real part of their user base. Short of a press release from google confirming the behavior, the average user is not going to go through the hassle of finding a replacement provider and then changing every email address associated with every service they use (if it's even possible for the service) all for some nebulous and impossible-to-confirm conspiracy theory put forth by tech experts and security experts that don't get listened to even when there is proof. And it's not like other companies and services would take a stand by ceasing to offer service to customers with gmail accounts- that's an incredible way to lose a vast portion of your customer base.
I wish it weren't this way, but I just can't see that bringing down gmail. Especially not when every google service requires a gmail account- probably the same one most people have had for years.
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 will launch a 'secured sender' program where you pay Google a monthly fee so your company's email doesn't fall into spam.
It hasn't kill gmail. How could it unless other providers refused to route gmail emails the way google refuses to route your personal server's email.
They could decide to randomly throw every 5th email that is not a gmail into spam and blame it on other providers having low reliability. Make it random enough to gently encourage you to get a gmail account again.
These are extreme examples, but Google could easily do these kinds of things and there is nothing any of us could do whether we use Gmail or not; it will affect us because they control so much.