Ren would’ve sensed that Luke’s life had come to an end so he knows that the only “Major person” to defend the rebellion would be Rey (If In IX Leia makes no appearance). However Rey is not fully trained to ren’s ability and has no trainer, even though their duels have been really close with rey sometimes coming on top, theoretically Ren should win. Another thing was the heat between Hux and Ren. This could be foreshadowing a civil war or split between then FO with Hux turning on Ren. That goes without saying rey is strong with the force which ,like all movie endings, will make a happy (ier) ending.
In space currently the rebellion have only the millennium falcon so if a squadron of tie fighters somehow found them then the chances of their survival are slim.