Ummm I think everyone saw what you saw - it's not rocket science. The Bears drafted a back, signed another one (who they aren't using now but whatever), so there was certainly a risk that Cohen would regress in terms of production.Ummmmm because you called it hindsight when, at least with regards to me, it certainly was not.
I think though that it was pretty uncertain how the work was going to be divided up and there was certainly a case to be made that Cohen's role as basically a WR (which occurred in week 1 if you recall) would be minimally impacted by the additions, assuming the Bears continued to make Cohen a focal point (you know, rational coaching and all that). Plenty of us didn't trust Nagy to do this, as evidenced by the last playoff game last year. The Nagy quote though about 'using him too much' was about having him do too many different things, which isn't quite the same.
Cohen has actually been on the field for a higher percentage of snaps this year so far so the presence of Montgomery hasn't impacted his playing time negatively, which I think not many people realise.