Who says Wentz would have signed as a back-up to Stafford earlier in the year? Former starting NFL QBs rarely are satisfied with back-up roles if they still think they can play as a starter in the right circumstance.
Often QBs will wait around to see if a better option...a starting option...opens for them like it did with Mayfield because some team loses their starter. And, it also comes down to money.
I'm not saying the Rams couldn't have done a better job at back-up QB. In fact, this has been an issue during Snead's and McVay's entire tenure. But, to just hammer the Wentz choice or think they could have had him at any time assumes Wentz would have come had the Rams called.
