No reason: it’s already been confirmed he considered firing Mueller and even directed McGahn to fire Mueller. McCabe justcstepped down today and Congressional GOP members are no joining on the narrative of trashing Rosenstein’s integrity.
Neither McCabe nor Rosenstein report to Mueller. Nor do Comey, Strzok any longer, Page, Schiff, Lynch, Bill, Hillary, nor Obama, for starters. Noone is directly trashing Mueller's investigation, although it will be impacted - Fruit of the poison tree.
Why is Rosenstein being trashed? Could it be that he is an Obama holdover that approved the use of an FSB propaganda paper for use as evidence in FISA court? I think that'd be worth being trashed over.
Matters outside of collusion: the investigation already had that latitude - the appointment authority of the SC has already been documented and argued in this forum.
And certainly not settled on the side you are arguing, at least yet. A court will determine that on Manafort's behalf.