Given the choice most users, in my experience, want a migration that means the minimum work for them. So a migration that preserves their existing emails and folders is the ideal. Anything that involves having two systems live at the same time is likely to confuse some.
Having had to migrate thousands of users from one email system to another, and from one server operating system to another, I find that it is very difficult to please all the people all the time. In the current situation I would say that it was more likely that the fault lies with Sky/Yahoo than with Google.