This is actually part of the Office 365 strategy where major new features are also introduced during the main lifetime of the current version, as we saw with Office 2013.
In the old strategy, these features were held back for the new version.
While you can’t directly change the sync order, you can still influence it by selecting the folder that you want to have synched.
Office 2016 will get the same treatment so future updates can introduce new features as well.
Until then, there are still plenty of new features in Outlook 2016 to use and discover.
Upgrading to Outlook 2016 from Outlook 2013 is easy and you’d probably need very little time to adjust yourself to it.
In fact, if it wasn’t for the new default “colorful” theme, you’d hardly notice the upgrade to Outlook 2016 at first sight.