It shouldn't be much of an issue at 4A schools. At my son's former high school, I don't think they have had a baseball player who also played wrestled or played basketball in at least 10 years, and usually not over 1-2 per year who play football. There is definitely more potential for issues to arise at smaller schools, which often need multi-sport athletes to be competitive. But, the coaches at smaller schools tend to be more close knit and more willing to work together.
Based on the news report, which is pretty sketchy, it sounds like you're looking at two windows of opportunity, Sept 7 through Nov. 4, and Dec. 9th through Jan. 15th. (I assume the summer is also open, but that's hard to tell without seeing the final rule). Imagine how much skill development you can do with an extra 3 months of off season practice! Now you can take that 6' 6" kid who has never pitched before and try to turn him into a pitcher. Or turn that kid with blazing speed into a hitter. Or take that utility player and turn him into a catcher.
There will no longer be any excuse for not having your kids on an off season throwing program because you can supervise it personally. (Really no excuse for it now, but some guys just don't/won't do it). No more sore arms on the second day of practice. No more waiting weeks for guys to get their arms in shape.
What I will be interested to see is how many coaches will take advantage of the opportunities. It will help some, and hurt others.
Let's face it, head coaches aren't paid enough. Assistants even less, if at all. Most don't do it for the money, but there are some that do. Those are the ones that will hate the rule. They're not going to want to log a bunch of extra hours during the off season when they're already making minimum wage.
If I'm an AD hiring a head coach I'm going to look for a young, ambitious guy who's willing to put in tons of extra hours to build a program. If I'm a head coach, I'm looking for assistants with that mindset.
Until we see the final rule there are still a lot of unanswered questions. Does the change apply during the summer time or just during the school year? Will coaches be allowed to coach their players in games during the off season? If so, under what conditions? The scary thing is that the rule will be written by some SCHSL committee, which frequently results in some ambiguous jibberish that nobody fully understands.
Should be interesting.