No way those players who believe they will be drafted will play spring ball. That is a risk they will not be willing to assume.
I have a 16 year old granddaughter who just finished her sophomore year in HS, and is being recruited by some D1 schools for volleyball. If she signs, no way she'll play ball after her Senior HS year of volleyball.... it's a fall sport....(that is, no Club ball that goes from winter through spring). Her college won't want it, and she won't do it for fear of being injured.
It'll be exactly the same for those players who think they may be drafted, or who might be UDFA signings. And that's a lot more players than who actually are drafted or who sign as UDFAs.
So while there might be spring College football, the teams' best Juniors and Seniors are unlikely to play and you have to wonder whether they can even field decent teams and whether it would be worthwhile.
I think those colleges opting out are just kicking the can down the street and offering some hope for their athletes when they talk about possible spring ball...... knowing that the odds are slim.