Why not? RingCT in Monero wasn't a thing back in 2016, and it got implemented in January 2017. XMR amounts hidden, boom. More privacy gained. Bitcoin could have optional privacy with softforking to confidential transactions and ring signatures. It's just that we want it to enter the system like a trojan horse, therefore we can't risk with stuff like that.
Like a Trojan horse obviously what the rest of us expect as any addition seems to be unnecessary and users become skeptical about additions to the network as soft forks, which is seen to have no value as things will always return to normalcy with time based in on-going activities.
It won't solve anything for the same reason people are still using P2PKH/P2Sh addresses even though we added SegWit many years ago and it offers a lot of benefits.
An opt-in option is not going to help much with privacy specially in a world where centralized places are rejecting CoinJoin transactions (opt-in) they can reject the said opt-in privacy feature too hence forcing its adoption to remain low which means it won't solve anything.
While it doesn’t solve much, if their is a chance that it could help provide more options, then it could have been something what trying but, as complicated as humans would be especially given the adoption of innovations and policies, most times it’s best to live things as it is and have people build a behavioral pattern about it.