Completely dedicated to God.

People would not get sick, crimes would not be committed, athletic events would be held on other days, cars would not break down, etc.

But those things happen. Now, the degree to which these are "unavoidable" or "necessary" varies from member to member of the church.

However, a leader from the church who was a surgeon that had to work on Sundays gave a talk in General Conference (twice a year, 12 hours of church talks given) about the Sabbath that I thought really clarified the "Spirit of the Law," rather than "the law":

"How do we hallow the Sabbath day? In my much younger years, I studied the work of others who had compiled lists of things to do and things not to do on the Sabbath. It wasn’t until later that I learned from the scriptures that my conduct and my attitude on the Sabbath constituted a sign between me and my Heavenly Father.12 With that understanding, I no longer needed lists of dos and don’ts. When I had to make a decision whether or not an activity was appropriate for the Sabbath, I simply asked myself, “What sign do I want to give to God?” That question made my choices about the Sabbath day crystal clear." ~Russell M. Nelson

https://www.lds.org/general-conference/2015/04/the-sabbath-is-a-delight?lang=eng

So, as a university that (for better or worse) often has to "represent" the church, BYU has decided that they will avoid athletic participation on Sunday as much as possible. Now, if a BYU football game is so late that it runs past midnight on Saturday, does the team just freeze and walk off the field? Of course not, I think almost everyone would agree that would be ridiculous. But Sunday play is avoided. . .now if a U.S. law was passed stating that all college football games were held on Sundays, would BYU drop football or allow Sunday play? I don't know.

But, as someone who avoids watching sporting events on Sunday (personal choice), I hope we never have to find out.