You're looking at it with rose-tinted glasses. Sessions 'offered his resignation' and was told to by Trump. It'll be same story with Rosenstein.
Rosenstein has been leant on before and held out. He’s a Republican but he’s also a career politician and still has years left in him. If he’s a witness to obstruction of justice and he knows he would only be retaining his position while his superior carries out yet more obstruction of justice... it’s not going to do much for his career.
I’d bet good money he’s going to be called as a witness.