RBR is still his best shot at winning more titles, IMO.
Mercedes won't consider him as long as Lewis is still driving (and this would have been the case regardless of what happened at Abu Dhabi) and it remains to be seen how strong Mercedes' intent is to remain in F1 once Lewis retires so might as well secure a long-term commitment now with RBR.*
* - Not that Mercedes only races because of Lewis, but once he retires, what is left for the team to prove? Like Audi with WEC, I can see them folding up things and moving resources to bolster their Formula E and other programs.