“Prigozhin is not part of the Siloviki. Should the military and FSB they view him as a threat, when Putin leaves power they can deal with him.”
Prigozhin has his own military powerbase with his Wagner group, which is growing more and more powerful and influential. Putin greatly appreciates Wagner’s efforts in Ukraine. Wagner group does not cut an run in Ukraine. Wagner just opened up a spiffy new modern HQ building in St Petersburg. Wagner group is growing in number of soldiers all the time. They are becoming a parallel military to the regular Russian army.
In short, Prigozhin is insuring his untouchability by the FSB etc. He is one smart calculating cookie.