I'd say most people lose their wallets to carelessness; with that said, if carelessness is a person's weakness, an online wallet would likely be safer than an offline wallet, but a properly maintained offline wallet is still preferable to those who want the extra security.
An online wallet is not going to protect you against carelessness. Invalid argument.
Not all carelessness, but having someone else worry about a secure wallet leaves you with just worrying about 2FA and a strong password; a server owner is going to be much more worried about keeping his computers free from malware than the careless person.
If your computer has malware it doesn't matter if you use a offline or online wallet. It's only a matter of time before the wallet providers gets added to the list of Paypal and banks.
I work in the field. Believe me, there are very sophisticated malware that bypasses 2FA by swapping out requests.