I think you have it quite backwards, and the RR has been retained for specific reasons that maybe don’t make him a white hat but will make perfectly good sense once all facts are known. But I’m content to withhold judgement and wait and see. Clearly Trump did not fire him and I’m of the opinion it was for pretty much the same reason he did not interfere with Mueller. Optics are important.
Most likely, Trump didn't want to be accused (falsely) of obstruction. So he leaves RR alone until Mueller is finished. That's all.