A big problem seems to be that the signatures are growing as the public key ages. After Merkle tree leaves are full, the remaining one would be used to sign the

A big problem seems to be that the signatures are growing as the public key ages. After Merkle tree leaves are full, the remaining one would be used to sign the next Merkle tree, and when that is full, the next one. Thus verifying an aged public key signature would require verifying the signature chain, which authenticates the current Merkle root. I wonder if this would be the only signature primitive, how the internet infrastructure could have worked out. Having a large number of leaves for a Merkle tree seems a prerequisite. But then, to limit verifying aged keys, some form of key renewal authorities would be needed, which would be trusted by verifiers. But at that point, would it be better to use MACs?