The most practical problem is that cash payments backfire: the moment you pay people to take a vaccine, you're telling them it's a chore that needs a bribe, not a safe, routine shot. That feeds the exact distrust causing hesitancy. People think, if this vaccine were actually safe and effective, why would you have to pay me to take it? You're spending public money to make the problem worse, and you get a bunch of people showing up for the cash who don't trust the medicine, while the genuinely hesitant feel confirmed. It's a bad deal with a built-in credibility cost.