I bolded what I feel is the main issue. You stated that because we are talking about our team's 4th/5th/6th CB playing the dime such as PJ Williams , then the performance makes sense to be progressively worse.
However, here is my issue.
With the Saints D, and their playbook, that team's "4th/5th/6th CB playing the dimeback such as PJ Williams" SHOULD then be just as good as the team's 1st/2nd CB because we run a lot coverages where there is more than just 2 DBs out there most of time. By that logic, shouldn't we then place asn importance on upgrading that 3rd CB if we're going to run majority of our plays with more than 2 DBs out there?
You also stated that there is a reason why 3 and 4 WR sets are getting more common--Doesn't that just further my concern and warrant the need to get a great/upgrade at the 3rd CB?