That is simply impossible because Bitcoin was not designed that way. If we add privacy features into the existing protocol (eg. though some weird soft fork) that won't fix anything because opt-in privacy is not improving the whole system.

The alternative is a hard fork that fundamentally changes a lot of things about the Bitcoin protocol. Why break what is already working fine and risk the whole system crumbling down? Most importantly why do it when something like Monero already exists?

A better solution is to work on privacy improving techniques that are more decentralized and get people to use them more. It would still be opt-in and won't improve the whole system but at the same time it won't require any kind of fork and change in the protocol that could break things.