It doesn't matter how careful you are, if you're the last human in a world of zombies, you're going to get eaten sooner or later. You may not have noticed, but one of the last humans not using gmail - that's me - recently joined the brain-seeking horde.
I didn't set out to be That Guy Who's Not Using Gmail. It sort of happened by accident. When Gmail came along, I was already using a hosted webmail service and at the time I thought that it was a more mature product. Maybe I was right at the time, but probably not for long after.
Then I became aware of of the unsettling facts around Google reading all your email: the creepily invasive "helping" apps, the probable selling of my data and likely use of it for ad targeting or who knows what else. I was deep in the Google ecosystem but I resisted letting them have my email. That was too close to home, too personal.
Then somebody clued me in. It doesn't matter if I don't use gmail; if most of the people I correspond with are using it, then the googles already have most of my email. It's sort of the opposite of herd immunity. I folded like a cheap lawn chair: reluctantly and with a squeak.
Complementary assets (app ecosystem, android integration, etc.) and oceans of cheap storage got me in the end.