Pitching prospects are high reward, high risk, but many have developed into solid MLB pitchers. MLB history is littered with pitching prospects who were traded who turned it into quality MLB pitchers. John Smoltz and Josh Hader are two extreme examples that quickly come to mind.
The Reds have been rather successful over the last 20 years in trading for pitching prospects. Harang, Volquez, Disco and Castillo were all acquired via trade as prospects. Harang and Disco were rated well below what both Williamson and Phillips are rated currently.
We can look at the past and come up with a general understanding of what pitching prospect are worth, what on average their MLB production will be over the long haul to teams. That is what determines their trade value. It’s not made up or pulled out of thin air, but based on decades of data. That data tells us that Williamson and Phillips are worth far more than 5 wins of trade value right now. They may never become MLB pitchers, but that is their trade value.