No, most of them really shouldn't. High schoolers who chose not to sign do so in favor of a D-I scholarship (in baseball or another sport), at which point they aren't eligible again for 3 years, or pro hockey. Many of the college juniors who aren't openly signable are deciding between another pro sport - presumably if they don't take the baseball contract, they choose the other sport.
The only group that in theory could come back the next year would be the JUCOs and college juniors that just said they were going back to school.