You are factually wrong about Russia. The new constitution removes the loophole used by Putin which restricted presidency of a single person by two consecutive

You are factually wrong about Russia. The new constitution removes the loophole used by Putin which restricted presidency of a single person by two consecutive terms. Thus in past he has used Medvedev as a seat warmer for one term, while holding most of the real power. Because it's a new constitution, his old terms got "reset" (it's quite debatable whether such reset is legitimate or not). In theory, after the new two terms he will not be able to get a third one and any future Russian president will be limited by two terms similarly to the US (well, unless of course the constitutions does not get changed again...). Also, you call the US constitution unchangeable? It's quite laughable. Not only there is a bunch of amendments, but real power structures have changed as well. Starting from the increased size and power of the federal government as opposed to what was envisioned by the Founding Fathers, following by the two terms restriction on presidents, and ending with the creation of the central banking system, which arguably goes against the spirit of the Constitution.