What could happen is another consensus layer that gave the right features and supplemented PoW was added to Bitcoin or people just used it with the Bitcoin protocol. This may happen in the future.
What if you had a TaPoS layer that had 1-10 second confirmations which worked with the miners where peoples wallets reflected both the ~10 min confirmation and the 1-10 second ones for added security and to allow for on the sidechain transactions to occur while not giving up on PoW security.
Would it help mitigate 51% attacks?
Depends upon what you mean by a 51% attack. It would not protect PoW layer at all directly but would protect the end user because after you receive payment you could depend upon the combined trust(with both the strengths and weaknesses of both consensus algos) of both systems. So If a 51% attack was occurring on the mining pools and the nodes on the TaPoS blockchain were not confirming than one could delay accepting and trusting the transaction until the community figured out what the problem was.
This brings up many other complicated questions though like if it is an act of supererogation and introducing new potential attacks but overall the benefits could be really nice as it would bring a lot more decentralization back into bitcoin, increase security overall, allow for decentralized instant confirmations, and act as a backstop against the asic manufacturers or mining pools from any mis-behavior.