I just found out that a bureaucrat is able to degrade a sysop. This way the bureaucrat can easily kick the sysop out of the wiki if he wants...
This might make sense - but not if there's only one sysop (like in my case).
Is there a way to prevent the bureaucrats from degrading the sysop?
Yes, you can select which user groups bureaucrats can set or remove.
For this, you need first to remove the "userrights" right from the bureaucrats, by setting the following in your LocalSettings.php file
$wgGroupPermissions['bureaucrat']['userrights'] = false;
and then set which group bureaucrats may add or remove with $wgAddGroups and $wgRemoveGroups:
$wgAddGroups['bureaucrat'] = array( 'sysop', 'bureaucrat', 'bot' );
$wgRemoveGroups['bureaucrat'] = array( 'bot' );