If all of these guys leave, you go from maybe a 7 or even 8 win season (and a perceived break through) to looking at 5 wins, and panic about the lack of progression.
Let's just take a step back off the ledge here. Yes, losing Moore, Gray, Prince, and Jackson would create some big holes for this team. But kissing the season goodbye 9 months before it begins? That's ridiculous. It's not like they have a bunch of scrubs behind these guys on the depth chart.
If Moore leaves, you rely more on Taivon Jacobs, who had a great finish to his 2017 season and showed a real knack for finding the end zone. You've got a bunch of talented slot guys: DJ Turner, Rayshad Lewis, Anthony McFarland. You rely more on those guys while you develop some of your younger, tall receivers on the outside (Jayden Comma, Carlos Carriere, etc.).
With Jackson gone, you have 3-4 guys who will compete for his job. RaVon Davis and Antwaine Richardson have both been impressive when they've gotten on the field. Deon Jones is a former 4-star recruit, and you may be able to retain Boykin, another 4-star guy.
If Prince leaves, you've got former 4-star recruit Marcus Minor ready to step in, as he did this year when Prince got hurt.
If Gray leaves, you'll have 3 guys competing for his job. You've got the 4th-best JuCo OT coming in this year's class (6'8" TJ Bradley), former 4-star recruit Jordan McNair, and 6'7" Tyran Hunt.