I've done a bit of quick maths (which might have holes in it) on the financial reality of waiting a year. Realistically, in order for it to make any sense to wait a year to sell Stones, the fee would have to be more than £60 million (assuming the £35 million valuation is remotely accurate).
Based on the new TV deal, Everton's gross income is set to increase ~33%, putting Stones' valuation somewhere around £45-51 million next summer, although that doesn't factor in the percentage increase for other clubs. Conceivably, Stones' value in comparison to other players could be closer to £55 million.
That speculative £10 million surplus vanishes quickly:
1) Other European clubs are likely to increase their transfer fees by English clubs' profit boost (as opposed to gross income boost), because that's their best bargaining position. They'll only care that Everton are making £40 million more per season and try to get as big a cut as possible. Without time travel, right now is the best time to for Premier League clubs to buy players from outside of England.
2) Instead of negotiating contracts for incoming players this year (again, before the increase in income) and holdovers next year, Everton will have to find a way to do both in one summer whilst dealing with the wage increase cap.
The true benefit to waiting until next year is in having the ability to arrange incoming transfers well before the season starts. From a financial standpoint, the best case scenario might have been a bid between £35-40 million on July 1 of this summer.
Regardless, keep Stones and win the league.