The idea that they have a monopoly in email or video hosting is ridiculous. They have a thousand competitors and it's easy for customers to switch.
They still have a huge slice of those markets for the same reason Microsoft used to have a huge slice of the browser market -- they're the default on the most popular OS (once Windows, now Android). But Mozilla et al took a huge chunk out of Microsoft's browser share after customers got sick of all of Microsoft's anti-user behavior.
Because people will stay with the defaults unless they have a reason to switch. But if you do give them a reason, like banning legitimate content they want to view and create, that's another story.
And the more people switch, the better the alternatives get and the easier it is for more people to join them.
except that due to youtube and google's near monopoly on consumers, none of this is going to happen. This is why google can choose to censor (or in fact, do whatever the fuck they want), because they know their customers (i mean products) don't have anywhere else to go to.