I think there's a lot of reasons to like how the mission age change can benefit the program. An 18 year old HS prospect indicates the intent to join the team, then prior to any enrollment goes on the mission for two years. Coming back at age 20, they can either take a redshirt year (to work the scout team, get accustomed to the system, get accustomed to college life, etc.) or a grayshirt year, depending on twhen they arrive back. At 21 they begin playing and have hopefully developed a degree of life stability that they will be ongoing contributions to the team for four straight years, not to mention the stability of working toward their degree without disruption. If they don't use any redshirt year following mission, then it allows the chance to use one later if there's an injury. Burning it before the mission can mess with that opportunity.
I'm sure there's others out there who would agree. The two year absence, book-ended by college enrollment is hard on many levels. Here's just one. I left Tulane after my freshman year to serve, then came back my sophomore year and when I took a math class I felt like a dummy. I hadn't even thought of math for over two years by that point and the learning curve hurt. I think I would have rather had the mission completed, and started college and had that adjustment time all at once rather than go to college, leave, come back, readjust, keep going to college. That pathway kind of sucked.
So, I think the age change helps with streamlined contribution to the team. It helps with streamlining the college experience. And, here's the one that will boost the missionary numbers--the streamlined socialization of leaving home at 18. I imagine that the one year disruption had opened the door for many young men to opt for other routes in life and not serve. Regardless of opportunity to screw up worthiness--just going and entering college or working or whatever for a year just opens up opportunity for other aspects of life. In the words of Michael Scott, it's a win-win-win scenario. Athletics, education, life. So yeah, go church for doing this.