I try not to trust blindly, although almost every inference is based on (to us) unverified information. I have read a lot, from any ‘useful’ source (I.e. not pundits). There were a lot of ‘dots’ outside the dossier, allinvolving Russia in some way. Are they true and/or connected? Don’t know. But in my layman’s opinion iI think enough to justify an investigation. You do not.  No right answer with what we know. 

But if they don’t investigate, we’ll not only won’t know if there was malfeasance, we won’t know if the investigation was politically motivated.  If there is something there, it would seem to be a valid investigation. If it’s all, or in particular the dossier, is made up, I will change my opinion.

The indictment certainly doesn’t make me change my opinion that there was something there to look at. Manafort (my guess) was one of the dots predating the dossier.