Firstly, it is quite difficult to manage so many usernames and passwords, for non-tech-savvy people such as my parents.

Secondly, don't you think that actually maintaining it is very difficult? One inadvertant click could expose one of those accounts to another with a different service provider, etc. Arguably, it can work should you define a different browser profile for each of those accounts!? Nonetheless, it is neither easy nor smooth, in my opinion.