And won't be because it "opts out" those receiving the funds you won't be paying. No realistic proposal anticipates that the people who have been paying the tax will willingly let the government break it's promise.
There are many realistic proposals to get from where we are to where we want to go, yours hasn't shown up because it is not realistic.
I think it is realistic. True, because of the flawed set-up of Social Security (pay-as-you-go), there would a short-term shortfall in funding. In the short-term, pull funds from the general fund to make up the difference. In the long-term, this goes away, the Social Security crisis is behind us (or, at least, is not a "crisis" for those who don't want Social Security in the first place), and Americans have more freedom to choose how they want to finance their own retirements. What's wrong with that?
And won't be because it "opts out" those receiving the funds you won't be paying.