I would agree with this thinking, especially this year with more CB's who have the ability to play the safety position in a pinch, due to injury or production issues.
6 CB's and 4 S's is one way to look at it, as is 6 CB's and 5 S's--especially seeing as how we have the Slater issue too.
That's all beside the point though...I would still respectfully disagree with the notion that Allen is a lock on this team regardless.
TC will be a time for it to sort itself out, but adding a 34 yr old CB who successfully defended 3 passes in 15 games played last yr doesn't bode too well in my opinion. It was a signing that was cheap and added competition for the roster. Why anyone would assume that Allen is a 'lock' is beyond me.
As noted by myself and others, many will not agree on who will make the roster--so many of these exercises are futile.