The pipermail UI isn't perfect (the monthly segregation can be annoying as you point out), but at least it has a synthetic and easy-to-navigate tree view.
Pipermail is *horrible* and it’s tree view makes things actively harder to read in large part because once the depth of the tree gets beyond like,, 3? or so, it just gives up trying to make it a tree and starts rendering all descendants past a certain point as siblings in a nonsensical order making it impossible to follow along on a discussion as everything ends up out of order.