What if we go big and say Kyler Murry the first overall pick in the 2019 NFL draft. He skips in NFL draft altogether and plays with the XFL for one year. In effect, he would be a free agent after one year and be able to sign with any team at any price. No rookie cap restrictions! He could name his price and pick his team rather than potentially being stuck with a bad organization. He could “survive” one year in the XFL with an XFL salary of $250,000 plus endorsements he can now get because he is not in college.

This isn't how it works at all. Every future NFL player must go through the NFL Draft process before becoming eligible NFL players, to regardless if they have spent time playing in other professional leagues. Then if that player is drafted by a team, that team has their rights for the duration of their rookie contract.

For example look at Jim Kelly, who when he was drafted by the Billls opted to go play for the Houston Gamblers in the USLF instead of going to Buffalo. When the USFL eventually folded, Kelly did not become a NFL free agent. He had to go back to the Bills as they had his player rights.