None of those 4 QBs careers have gone anywhere. They have been perpetual losers and really ought to be (and probably are) on their way out of the NFL.
On the other hand, Wentz, despite his flaws, has been an NFL winner, played and succeeded in Playoff games, is only 31, is completely healthy, and has a leg up in learning the Rams O system. There are rumors about him not being a good teammate, but I haven't heard anything that makes me think that's actually the case. I suspect he sat at home because his salary demands were too high. Reality is a b!tch.
None of us know if McVay likes him in the QB room or not.
None us know if Wentz would accept another fairly cheap year of being a back-up.
But, if McVay is good with him, and Wentz is OK with being Stafford's back-up at probably something like 2 years for $6 mil plus incentives for playing time, then I think he's the right guy.
I'm sure he'll hope for better offers. But, $3mil a year is a LOT of money no matter who you are. Lots better than zero.