Is that what they really announced? Dave Rose, Bronco Mendenhall, and every other athletics coach at BYU has to be smiling. This is a huge boon. There is less mystery to the missionary process now. You are almost going to be 95% sure whether a guy is going to leave on a mission now or not before they even come to BYU. You could have guys graduating early from high school in January, going on missions, and then back when they are 20 for four straight years of athletics training. Instead of lolling around at BYU for a year redshirting, then losing most of the benefits of the training from that year prior to the mission.
And on the church front, there is little difference between 18 and 19 year olds, except that you're probably more likely to get 18 year olds to go on missions. The guys that usually don't go are either those that have never wanted to or those that waffle around for awhile in indecision after high school. You plug them in right after high school and you're bound to get more missionaries out there.
I think it's good to get missions out the way. I've never liked the idea of....starting college...getting serious girlfriends....leaving college and girlfriends for two years thing. Much better focus if you don't always have something that's breaking your life path up.