Signing Suh is a win for the Rams no matter how the Donald thing turns out.

If Donald grows a brain and reports or takes the offer, then the Rams have the best DT tandem in the league.
If Donald continues to show more greed than sense, then Suh is about as good a replacement as it gets. In fact, if the Rams get a great trade offer for Donald they can take it, keep Suh for a couple more years for a lot less than signing Donald, and still have a championship caliber D.

Having Suh has already given the Rams the ability to not panic and let AD stew.