Because there's no such thing as no downsides.
Discussion
That's literally the OP's premise.
Yes, that is the premise. There are no downsides to this hypothetical question. If you could have instant, private transactions with self-custody Bitcoin, and not lose a single thing that makes Bitcoin decentralized and secure, that sounds like a good thing to me. I kind of thought that’s what we all want.
It is what we all want. Don't let laser eyed religious zealots convince you otherwise. Bitcoin is not perfect, can and will improve.