Why Is Medvedev Worried About The US "Destabilizing" Russia?

Former Russian President Dmitri Medvedev gave the US an unusual warning: Getting rid of Putin would make things more dangerous rather than less.

"This means that Russia must be humiliated, limited, shaken, divided and destroyed," Medvedev wrote, saying if Americans succeed in that objective, "here is the result: the largest nuclear power with an unstable political regime, weak leadership, a collapsed economy and the maximum number of nuclear warheads aimed at targets in the US and Europe."

Such a warning necessarily implies that such a regime change in Russia by the US is possible. The danger of an American success would be irrelevant if there were no chance of success.

Dmitri Medvedev is hinting that Putin is vulnerable. Why would he do that?