XRP is an extremely risky crypto-asset. It is highly risky because of its centralized nature. The vast majority of the total supply is not in free-float. If any of the promoters want to dump his holding, then it can destroy the exchange rates. Ever since I entered the world of cryptocurrency, I have stayed away from this coin. There are far better cryptocurrencies out there to invest.
This is actually something to look at on this coin. What if one of the owners decided to leave and sell his share? This scenario is very possible to happen. I like how cheap XRP transaction fee is and how fast it is. But it is the team managing this is quite the scare from crypto users. Because it is centralized, you'll never know what will happen to their coin once there's a change with their management.
Well.. the promoters can't sell their share in one go. There is not enough liquidity in the market to do that. If you check the daily trade volume of XRP, then you can find the numbers are lower than that of many of the altcoins outside the top-10 list. The promoters may be dumping their holdings, but they are doing it very slowly and using multiple exchanges.