If by "everyone" you mean the large self-voters who are currently voting at 0 minutes, it isn't quite so simple. If they switch to voting at 15 minutes, another user chasing curation rewards can simply upvote the post at 14 1/2 minutes - losing a small bit of the curation reward for voting slightly before 15 minutes, but gaining a portion of the curation reward for the huge self-vote that will come behind them at 15 minutes. In actual practice the optimal time to self-vote for a large account will likely be earlier than 15 minutes, which means giving a portion of the reward from the upvote back to the reward pool.
We already see something like that currently when it comes to users attempting to maximize curation rewards on auto-votes set on other user's posting - even though currently a vote at 30 minutes might seem to be the optimal time to vote since it gives full curation reward, in practice the optimal time to set an auto-vote on another user's posting is usually somewhere around 20 minutes or so - the game is to vote early enough to come in before other votes, but still vote as late as possible to maximize the % of curation reward.
Removing the 0 minute self-vote loophole will cause something similar to happen for self-voting.