A heavily armed SDF backed by the West would be a significant threat to Assad if it were to fight the government in tandem with the other rebels that could potentially overwhelm the SAA, but to get the SDF to join in on regime change would require a level of diplomacy that America has so far failed to demonstrate. America has offered nothing to the SDF in terms of diplomatic assistance as far as I am aware of to reconcile it's relation to Turkey in favorable terms that ensure long-term security and when it comes to who replaces Assad, no single rebel faction is qualified or cohesive enough to govern in a way that would stabilize the country and not pose a threat to the SDF. And even if empowered by increased Western support, the potential for any single rebel group to reach critical mass as occurred during the early stages of the civil war is unlikely with the public exhausted from war and uncertainty. As in Afghanistan, the rebels group ended up fighting each other and so in Libya whereas the US failed to ensure stabilization would occur post-war. In Central America and South America, despite backing proxy forces against left-wing forces, the left-wing eventually won elections and more or less secured power in many of these countries post-cold war.