There's no reason to think that Warren is taking a position away from anybody at this point.
I think this entire post was very good, and I agree with a lot of what you say.
I'm also in agreement on some level with your thoughts of Warren, the problem is---"who" would that guy be who he'd be taking a position away from at this point?
The Defensive line seems to be the weakest area on that side of the ball right now, and since it wasn't addressed in the draft (no knocks, b/c you can't take care of everything through one draft) we still have some holes to fill depth wise.
We may be able to sign a roster cut or two as the summer moves on towards the middle-end of August, especially due to the increased rosters. However, the jury would still be out on whether or not that specific player would know the system as well, + be as effective as Warren has been at times in a rotational role.
At this point in time, I am happy with the signing and would be in agreement with those who value his vet presence and ability to provide run support, knowledge of the system, good work ethic, and a healthy body. In my opinion, Warren is a bit old, but he has pretty much done everything asked of him and provides good rotational value and leadership.
If another player comes along, then as you said--all the better. Until that happens, Warren very well may end up making the 53 man roster.