Hang on a second. I'm not questioning your assessment that Benny can handle the RB2 role, because I simply agree. But what leads you to believe that the Rams weren't/aren't excited about Mason because of Cunningham?
I personally believe they are ecstatic to have Mason in their backfield. I definitely don't think that having Benny played any role in the drafting of Mason. Nor do I think that they felt any obligation whatsoever to draft Mason. We have no clue as to how far ahead Mason was as the BPA on the Rams board at that time. I believe they could have had deep discussions about multiple players; that, if, they were there for the taking, the Rams were taking X player.
We went from a "middle of the road" RB trio, to a formidable and stout one.