The Kingdom's mages didn't seem to be overly relying on the MC, rather than MC was so talented that he completed his work quickly and would clock out early, rather than sitting around and appearing to continuously work hard - which seemed like a critique of current Japanese work culture.
You could even argue that mc's treatment at work was his own fault to a degree, as he wasn't willing to bend his principles and pretend to be working.
So I don't see how, now, that mc is gone that the kingdom is suddenly suffering from a work shortage; they'd get someone less efficient, sure, but it's not like they'd be doing less work on a day to day bases which the kingdom was operating at.
(this is all assuming I didn't misread the first chapter, or that I didn't read a mistranslated chapter)
A more reasonable way for the kingdom to get their comeuppance would be for them to simply get btfo'd by the empire because of the MC, for them to note that he WAS a kingdom citizen and wondering why they lost such an exceptional talent.