I know, it’s kind of weird to get my head around, too. Take the transitional implementation years out of it. Under the old system, players had 4 years of playing time. Now, they have 5 years of playing time.
In a perfect distribution example for simplicity, that means 25% of the roster used to leave each year. Now, only 20% of the roster leaves each year. It decreases yearly roster turnover no matter how you look at it, which means coaches simply have less need to sign as many high school seniors each year.
Because teams are bound by a roster cap, they can't just expand the team to hoard both groups. It is a zero-sum choice. Furthermore, without traditional redshirts or medical waivers, high school seniors who are signed will be on the playing clock immediately. That means there is less incentive to take on high school "projects" because you waste a valuable year of eligibility just developing them.
Now, couple it with the portal. At the top levels of CFB, does a coach choose a group of developed 22-year-olds or 18-year-olds fresh from HS to fill that smaller 20% hole? Obviously the former. This creates a trickle-down roster displacement where the 18-year-olds have to go somewhere lower. The recruits who would’ve gone to Alabama are displaced down to Boise, and the ones who would’ve gone to Boise are pushed down to the FCS (rough examples).