The Only Way to Prevent Julian Assange and Other Political Prisoners from Getting Navalnyed ...

... is probably to make credibly clear that if it happens, the lives of all those responsible in any way, shape, manner, or form for their deaths (including by incarcerating them and/or keeping them incarcerated) are forfeit.

If Assange, or Ross Ulbricht, or any of a number of others, die in prison, the question is not whether they were intentionally killed, or whether they were just allowed to be killed or to die.

When someone gets put in a cage, all persons (up to and including POTUSes who don't pardon them) responsible for them being in that cage are also jointly and severally responsible for what happens to them there. They are, as demonstrated their actions, imminent and credible threats to the life, liberty, and property of anyone they approach, and should be treated as such.