It depends on the school. It's definitely something a school like Nebraska should consider, but a school like Bama will still hang onto the players they want, as long as they're starting, and so long as the coach doesn't leave. I think Saban said there was only one player who transferred out that he felt was a bad decision (I think it might have been Aaron Anderson). It's just depth that we lose, and I don't know how you fix that problem.
As for schools like Nebraska, Colorado had some success using only the portal. I think Deion is recruiting high school a little harder now, but he was barely touching it when his kids were there. The question would be if 9-4 is the ceiling for a team that was built almost entirely through the portal, because Colorado is really the only school that has tried it. That's probably good enough for Colorado, but I'm not sure Nebraska would be happy with that. They fired Bo Pelini for only winning 9-10 games per year.