I don't thing ABM defenses really change things. Russia can throw 2000 warheads at us. Can we stop them all? No way. But would we have a chance at stopping one North Korean nuke? Yes.
So I think this is only destablilizing if Russia and China decide that it is. If Russia is looking realistically at the situation, there is exactly zero chance that the US will willingly take the amount of damage that Russia could cause, even with ABM defenses reducing the damage.