The switch is not necessarily stupid, but it is too risky to implement.
John can transition to playing fulltime sentinel. Then Reduxx can be slotted in to play secondary duelist or secondary controller. But you would not be able to run double initiator comps which is very popular. So Reduxx would have to learn some initiator agents too.
SEN would be taking too many chances with the different role switches. If John's KJ, Vise and Deadlock don't look good, then they have blundered pretty hard.
Same for if Reduxx's initiator agents don't look good. Then they cannot run double initiator comps to a high degree.