I was thinking about this too. It occurs that what people want is money, power, prestige, a sense that they are valued etc. If you have an elite group (and it is an elite group isn't it?) like sorcerers, you hug them close. You give them money, a sense of prestige, importance, the legal ability to order other people around and so on. If you are loyal to me, I will be loyal to you... and then even if some go rogue and fancy the imperial throne, there's probably enough loyalists to fight them off. After all, you can only have one emperor at a time. They can't all be emperor, so the usurper has to start offering bribes to peel his colleagues away from the throne.
I guess that is why in my head the Mellandic throne is more vulnerable. One really talented guy teaches himself and tries his luck. He has no real peer competitor in the Kingdom.