Warpstones all share the same global countdown, so there isn't any really good reason to make them unstackable.

The countdown shouldn't be visualized with a durability bar, but for instance with an overlay gradient (similar to how ender pearls work).

Making them stackable would make it so much easier to craft waystones and sharestones with them.

Superseded by #683, there will be a new stackable item for the crafting recipes in 1.20.