Earlier quoted context omitted.
People get hacked and/or locked out of accounts surprisingly often. A good (and tech-savvy friend) was recently hacked, and managed to get thousands of dollars transferred before found it. PLEASE, PLEASE, PLEASE, if you have a Gmail account please follow Jeff Atwood's directions below. Turn on two-factor authentication, make sure you've verified a phone number and recovery email address account. http://www.codinghorr…
Suppose you turn on two factor authentication, and then something happens to the phone number you used for it. Does that mean you've effectively lost your Gmail account? (Not a rhetorical question. I don't know how or whether you can lose a cell phone number - as opposed to just the physical phone, which presumably shouldn't be a problem - but that doesn't necessarily mean it can't happen.)
Lastly, they will give you a list of ten one-time-use codes which you can write down or print out and put in your wallet/safe.
If all the above fail, I believe there's an account recovery procedure, which takes a couple of days and involves sending proof of ID.
So you're not relying on a single device, and certainly not on a single phone number, to be able to retain your account.