First, there are strong diminishing returns to well-being from wealth, meaning that moving oneself from the top 0.5% to the top 0.1% of global income earners is a relatively modest benefit. This relationship is well studied by social scientists and psychologists. Compared to the potential stakes of OpenAI's mission, the balance of importance should be clear.
Two, employees don't have to stay at OpenAI forever. They could support OpenAI's existing not-for-profit charter, and use their earning power later on in life to boost their wealth. Being super-rich and supporting OpenAI at this critical juncture are not mutually exclusive.
Three, I will simply say that I find placing excessive weight on one's self-enrichment to be morally questionable. It's a claim on human production and labour which could be given to people without the basic means of life.