It’s a little more cumbersome then that.
Work it through for a minute and imagine that 2.5, 2.5.1, 2.6, 2.6.1, 2.6.2, 2.6.3, 2.6.5 (.4 got skipped due to evil people), 2.7, 2.7.1, and finally 2.8 all had the upgrade button. Let that sink in while thinking about the current users who clicked from 2.7.1 to 2.8.
Now imagine that each of those releases had the magic downgrade button too. Take a moment, find your center of calm, breath and med-i-tate.
Still with me? Now support users who have admin rights, don’t read the instructions, and push buttons and want support and they keep hitting the upgrade or downgrade button.
The thought of that alone would get me to cancel my forum account.
Hosting your own blog is not for the weak of heart. You must develop some system admin skills to make sure that you can restore your backups, troubleshoot PHP and web server problems, and put everything back to the way it was. Patching releases is in that too and not just WordPress. The Apache, Mysql, and PHP components need to be maintained.
That’s what it means to host your own blog, you get the liberty of doing what you want but you also get the responsibility.
The automatic upgrade is nice and works for most people (really does). But it does not let people off the hook for admining and maintaining their installation.