I think many folks want to believe that prior to the creation of the transfer portal in October 2018, the really good players were staying put because they were "happy". In reality, most were probably staying put because their head coaches were making it very difficult for them to transfer since they wouldn't grant them a full release, or wanted to put conditions on their release (such as which schools they would not be allowed to transfer to).
The portal is working, IMO. Coaches should not hold all the power. Player should be able to move on as they see fit.
As for immediate eligibility, that's a separate issue. When the portal was created, there was no immediate eligibility in the main sports.