relying on permission subscriptions for this was unreliable (a permissible is not always subscribed to a permission even when it does have it), and also difficult to control (for example there have been various bugs in the past where a Player ended up subscribed to broadcast permissions when it didn't expect to be, thanks to permission recalculation happening too early).
In addition, we might in the future want to have broadcast receivers which are not permissibles (i.e. a more general interface than CommandSender (why does a broadcast receiver need to also be a command sender, anyway?)), which the permission system wouldn't be suitable for.
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