Message box has ever been useful for Apps in all times. PowerApps, which is Mobile First, is also made for tablet and web! So, It would be nice, actually important for PowerApps to support Message Box. Thinking outside the box, a Powerapp should 'know' the OS via which it operates hence uses that OS's genuine message box to pass to the user the meant message box, and then this will hit the spot!
One thing to note, please make Message box able to run a command after a message box has been closed.