Apart from the Haskell fun, xmonad is really really good. Especially how it handles workspaces on multiple monitors. By default, each monitor has one workspace. Not one workspace for ALL monitors.
So if your side screen has say documentation open, and you switch on your main screen from terminal to editor, only the main screen changes. The side screen stays untouched. Some people prefer it, some hate it. But it's good that xmonad offers it. It also has hot reloading of the config, which is also nice.
Sure, Niri's config is approachable, but is it hackable? It only offers what Niri offers, right? You can't extend its functionality by config alone.
That is where XMonad and others shine. Sure, they might not be for the "I don't wanna spend time configuring my WM to death" crowd (and I mean absolutely no derision in that. We all have different rabbit holes we want to customize to oblivion), but Xmonad or Stumpwm etc allow for those shenanigans, and I myself am very very happy to have those.
Long story short, it's not all 100% about the type safety, it's also a lot about the extensibility of it.
Niri is nice, not mocking it. But infinite scrolling is not everyone's jam. Some others might even question the choice of KDL instead of JSON without any clear benifit.
Whatever you prefer and if it works for you, great. That's why we have choices ^^