Why commitment transactions can't simply be HTLCs instead of contracts revocable by having the second half of the Revocable Private Key? I mean, why bother to create a Revocable Public Key from two halves of Alice's and Bob's private keys and when invalidating a transaction sharing the other half, when both Alice and Bob could create two transactions, both revocable with two separate keys known only to Alice and Bob, which later would be exchanged in order to invalidate old transactions?