Let's even be realistic and assume that even in this hypothetical, it's still not perfect, but it's way better than any of the traditional (non-voluntary) implementations. This is because as an alternative to coercive taxation, where only 20% of the funds taxed to power traditional basic income schemes, BIC investors enjoy a tidy profit.
In fact, in this hypothetical, let's assume no government tries to offer anything like a basic income anymore because BIC just does it better.
Anyone with a desire for altruism needs only to invest in and hold BIC. Want to see how altruistic someone is? Just check their portfolio.
So how can you be against Basic Income in this scenario where it's implemented like this?
In this hypothetical scenario, anyone against the general idea of Basic Income cannot use any pragmatic argument because BIC works fine.
They have to use purely philosophical arguments. In fact, there's an objectivist argument that non-producers who get hand-outs will have a lower self-esteem, which will damage them. That makes Basic Income, no matter how it's implemented, immoral.
But all of these objectivists must invest into BIC because it's profitable for them to do so.
Why? Because the objectivists are outnumbered by the altruists, and BIC made altruism profitable in this hypothetical.
Will an objectivist intentionally hurt someone's self-esteem by supporting a successful investment instrument?
Or will an objectivist intentionally ignore a successful investment instrument in order to dismantle the only working Basic Income implementation?