That's the thing, isn't it? For some reason Putin prefers to make a spectacle of this – take Litvinenko or Skripal. Intimidation, I guess. There are more effective ways of killing people, but when it comes to high-profile targets it's as much about making a point as it is about killing someone.
The fact that Navalny had lived is very much a coincidence – if you poison someone with the intention of this, very unique, poison, to start working when he's on a plane is very much a death sentence. The fact that the pilot had reacted so quickly and the doctors in the emergency vehicle decided to give him atropine was insanely lucky, you can't really count on that. There's also not much room to testing substances like Novichok (especially new ones – this is not a one particular substance, but a group of substances and the German authorities say that it was a different version of the substance) on people for obvious reasons.
Also – never underestimate the level of incompetence in today's FSB/GRU/FSO etc. Those are the people that decided that visiting Salisbury to look at "the tallest church spire in the United Kingdom" is a convenient excuse for the public world-wide interview of 2 murder suspects. Those are the people, whose agents kept taxi receipts with GRU's office address on them while on a mission at Rotterdam so that they can get compensated for their spendings after they return back to Moscow.
I loved this phrase by some Russian political reporter, I don't really remember who, but it goes roughly like this: every time a murder of Putin's political opponent happens, it never seems to be a convenient time for him... yet those murders keep happening. What's obviously most damning in that case, is the reaction of Putin and Russia. Omsk's hospital said that Navalny was in an insulin coma (and then proceeded to put up a dozen of equally bat-shit crazy theories neither of which explain what happened with Navalny); there's still no criminal investigation started on the matter by the Russian police. Can you imagine anyone collapsing on a plane with symptoms of neuroparalytic poisoning, falling into coma for multiple weeks and the police refusing even to start an investigation on the matter?