Two things, and these are both centered around the hypothetical of the club actively planning to sell Stones next summer.
1 - If you as an individual were going to invest a large sum of money, would you invest it in a 21-year old footballer, or would you choose to invest in a much safer commodity? Stones may represent a smaller risk than almost any 21-year old athlete in the world, but compared to anything else, it's still a massive gamble.
2 - The best time for English clubs to buy players is right now, before the new TV deal kicks in next year. It doesn't matter if Stones is worth £50 million next season, because every other club on the continent will be demanding higher fees for their players. The trend has already started, and it will only get worse. If Everton want to spend a significant amount of money in the transfer market, this is the window in which to do so.
The only way the reward ends up outweighing the risk in my eyes is if Stones goes from being one of the best to the best CB in the Premier League and commands a fee of over £60 million. That's a foolish gamble to take. If they're going to sell, from an economic standpoint they should sell now.