That solution won't work for those with multiple mailboxes.
I'm not sure what you are meaning by 'multiple mailboxes under the same Outlook profile'.
It will work for 1 mailbox per profile. Mine has 8 mailboxes in 1 profile. It's a pain to switch profiles as I am constantly using 3 mailboxes at the same time, with occasional lookup in 5 other mailboxes. See screenshot:

Ah, I gotcha. I wasn't following, but I actually do the same thing, multiple mailboxes (3 of them) in my main Outlook profile, available from the left Navigation pane. Anyway, to be honest, I haven't noticed that the ribbon tries to reset because I've had them locked down (read only) for so long.
QuickSteps was the only thing still causing me problems. Once I got rid of that and with my setting *.officeUI files to read only, my ribbons don't change at all.
I think it's absurd that Microsoft gives Outlook (and other office programs) the ability to customize ribbons, but hasn't fixed the issue of them reverting back. I have a feeling it's because the folks who actually customize their ribbons are the minority.