1. We dont need to see it to know it. Just like we dont need to see an in-game scientific report that the sky is blue(assuming it was) for the same reason it is IRL. That's just being spoonfed.
2. It doesn't insta kill you, you have to drink several doses of it to die. There's also comments from at least one NPC suggesting that they believe its the purified water killing them, though they believe the cause is something other then what it actually is, which means many would stay away from it.
3. I don't see it as a weakness. Siding with a group whose only goals to kill anyone who isn't them is a pointless addition because then there would be no more game to play.
4. Except the BoS is literally nothing like that, they willingly ignore threats they could easily take out, like the slavers of Paradise Falls, the raiders of Evergreen Mills, and Talon Company, they willingly fire on non-feral ghouls without care, and only attack super mutants because they were sent to negative them in the first place. This isn't even taking into account the several hints that Lyons only wants the purifier to use it as a PR tool to get people to join his dying order, and not because he REALLY cares. This "Lyons is this ultra saint" meme its ludicrous, and ran its course ages ago. Lyons is a self-serving [censored] like all the other BoS, and only helps others when it helps him, and there's many reasons you might not want a person like that in charge of the purifier. the only way in which Lyons is different is that he does a better job of hiding his true nature.
5. the LW already sacrificed himself to save the wasteland, why not again?