Five seconds to change your mind
Pressing send now starts a countdown instead of firing the email. Change your mind and nothing has left. You can also see exactly how your complaint arrives before it goes.
Sending a complaint is the step people stall on, and it is easy to see why. Up to that point everything is reversible. Then it is not.
What changed
You get five seconds to cancel. Pressing send arms a countdown rather than calling the mail server. Cancel it and nothing has gone anywhere.
Every check that could reject your complaint runs before that countdown starts - placeholders left unfilled, an address that will not accept mail, the acknowledgement. So you will never watch a timer run down and then get an error.
You can see how it arrives. Your complaint lands in their inbox as your name (via NoReply). From you, with your name on it, not from a company they have never heard of. That was already true. You just could not see it, and it turns out to be the thing people worry about most.
Cancel gets the visual weight over "send now", because someone reaching for that button is having second thoughts and the option they want should be the obvious one.