And I think you'll have your answer of what is going to happen:

You are going to need new facilities and upgrades across all sports to compete. In the past 10 years the U has built an indoor practice facility, a football center, a basketball center, renovated the Huntsman Center along with soccer fields, softball fields and more. This is just to get up to speed with other members of the conference, and despite all of that they still aren't. Millions and millions of dollars spent, and based on the guarantee of millions of dollars of revenue each year, of which you have no guarantee.

Coaches salaries have gone up across the board to be on PAC12 level.

Now with guaranteed scholarships, ongoing healthcare coverage, likely cost of living money (Texas is giving their football players, and presumably all their athletes, an extra $10k a year) and that is even more.

Now I don't sit on these boards but I know how business works with the church and I think being faced with doubling, tripling or quadrupling athletic costs, along with no guarantee of a return on that investment through P5 conference affiliation and I think it will be hard to get that one passed. They'll continue to ask you to do more with less.

In my ward there was a chair in the main foyer with a nasty old jeri-curl juice stain on it that I'd say did more to damage testimonies of members than a faltering BYU football squad. Plead, beg, pray, entreaty... nothing was going to get the church to replace that nasty old chair. Finally the facilities guy in the high council had to chuck it into a dumpster and pretend like nobody knew what happened to it to take care of the problem. This is the same group of guys who is going to approve the same things like the Utah Football Center? I don't think so.

I'll be completely honest with you, I have a hard time seeing BYU as a D1 athletic school 20 years from now. I struggle to see how Utah is going to keep up.

Also, since most of your argument hinges on point #1 being true, and assuming it is, the question that might follow is: Is there any reason we can't get all of those same benefits as an FCS school?