He cares about his interests, which may or may not be stability, or a certain kind of balance of power there. The important thing w/r/t our interests in Syria should be stability imo, not creating a vacuum there is more important (imo) than Russia maintaining an ally in Assad or port access or whatever combination of interests they have. That's a concession to them, but that's fine compared to the alternative- they are not friends of ours, adversarial on things pertaining to their interests, but they're not enemies on the level of regimes we've faced in the past imo, even if they try to monkey with our elections, which has been going on in both directions, and similarly with China, for a long long time.