Me too, but I actually think the complaint is totally valid. I mean, somebody does use it, and it isn't some horribly obscure feature, it's built into most terminals, it's pretty much standard Linux functionality at this point, and old REPL does support it, so when you introduce new default REPL, but then say "uh, no, we don't use that shit you use, so fuck off and use the old REPL, loser" it's not very nice. (And even if they didn't outright call him a loser in that thread, it's still not very nice.)
In short, the fact you, me and these guys don't understand why anybody used vi-mode doesn't mean shit. It came before us, apparently still exists, and well may outlive your neovim and PyREPL. Prioritizing other stuff is one thing, dismissing and closing the issue because "nobody (i.e., me) needs that shit" is another.