Why not go the other way and limit the number of portal transfers per season? If you limit the number of high school recruits per school to 20 or even fewer, then you are forcing schools to go to the portal more. I would prefer to see a limit on total roster size throughout the season and a limit on the number of transfers.
This would probably help to bring down the number of transfers because you would have fewer young players sitting on the bench. Sure, the 20 players signed by the big teams will still represent the bulk of blue-chippers, but those last 5 spots can be important.
Think about it like this. How many guys have we been battling up to NSD with Bama or UGA over the years? If they had 5 fewer spots, then we would be more competitive for those guys, as would Ole Miss, VT, UNC, etc.
It would just be better for the sport, I think.