Sorry for delayed response, I posted then went to sleep!
I don't broadly disagree with you, but if we improve our defence, we don't rely on the forwards as much, and vice versa. Ideally we have all the best players and the best team/squad - but we can cover for weakness through other strengths. That is where Slot/Hughes etc earn their money getting the balance right
Trent's defending at times has shown this. He isn't the best best RB but his other strengths meant we weren't as exposed, if he didn't have his passing skills - we would certainly have said we need an upgrade at RB