Realistically, if Google discontinued Gmail, people would move to other email providers. There's plenty of big ones.
BTW: Want to avoid being caught in that eventuality, but don't want to stop using Gmail? Buy yourself a personal domain name (for your last name or some such) on https://www.gandi.net/ and set up a redirect to your @gmail.com address. Your email is now john@smith instead of johnsmith@gmail. Congratulations, it looks a thousand times more professional, and you have it for life; if gmail dies, you can move providers without changing your email.