The transfer portal is not a fix, its a Band-Aid, and sometimes its a questionable Band-Aid. The transfer portal is a lot like JUCO, only a few great players come out of it and most never really pan out due to lack of eligibility and development. There is a reason most of them are in the transfer portal, and relying on kids who had no loyalty to the program they once served and trying to integrate them into your culture is not an easy thing when they have already started to be molded in someone else's image.
Most kids in the transfer portal are kids who aren't getting a ton of playing time where they were and are looking to play immediately. That's why most of them move down, and its good for teams in the G5 or FCS to get those kids and let them try and play. The rest of the transfer portal kids never sign, and that's around 70%.
That's why you can't rely on it and that's why building your team through the standard recruiting method is the best avenue to flip a roster. Sure you might get a piece or two in the portal, but its always going to be more famine than feast in there due to a host of different reasons.