Your whole first paragraph causes me cognitive dissonance, it's almost like you believed the Russians would still follow a treaty for a moment longer than it proved convenient for them to do so, or something. Have you never heard the phrase 'Broken as many times as a Russian promise?' Do you not think that six different things can be made with the same serial number stamped on them, and marked the same, as long as no two of them end up in the same place while anyone is looking?
This treaty is in Russia's current interest because it limits US but affords them more-or-less status quo for what they have that is actually still in working condition, thereby obviating any need on their side to put any more resources into the nuclear weapons money pit than they already do, while preventing us from obtaining any dominance over them during their current less-than-world-power doldrums for the next ten years. Not that our current strategy calls for dominating them, even if building them into a distant second place was the strategy that won the Cold War, because that wouldn't be very Progressive of us, comrade.
They expect to be a lot stronger in 10 years. They expect us to be no stronger than we are now. This treaty is a positive conservation of force exercise for them, a no-gain for us.