I don't think there was ever any need to. VX nerve gas will permanently inhibit the on/off switch for your entire nervous system, like weaponised Parkinson's or multiple sclerosis. It rots your brain while hyper-stimulating and/or paralysing every nerve in your body. The novichok agents they got Navalny with are several times more potent and it was a concentrated dose that he drank. After that he was walking dead in the same way a highly irradiated person is. I'm surprised he survived that long, much less with shitty prison healthcare.