It seems to me there’s a contradiction in your argument, or at least that it misses the point.
If we start work with a mindset of only striving for what we need, it’s reasonable to think people may not work has hard and be as productive to society as they could be and in fact be in less of a position to donate.
You fear that society may not be as productive if people only work for what they need, rather than what they want. Fair enough. You also assume that they’ll use the fruits of that extra productivity toward charity. But if they’re planning to donate much of their extra income to charity, that’s the same as striving only for what they need (because they’re donating the additional part that represents what they “want”). And if they’re only going to donate a tiny fraction (2–5% as you suggest), then it’s really only a token gesture that won’t help solve wealth inequality.