That is not possible. That doesn't work for Joomla. Joomla needs to know what component the URL belongs to and it does that with a Menu Alias, which also contains the view. The new SEF URLs in latest CB builds are as follows.
domain/menu-alias/username
If you're not seeing this then your menu item to CB isn't a valid internal Joomla menu item. If you were to take out menu-alias then Joomla has no clue where you're going, which means you'd have to hijack all 404 redirects and assume they're profile URLs which is bad SEO, but can probably be done from htaccess redirect/rewrite rule.