I was looking at an article on Oregon's portal players (around half of their starters were transfers, and all of the guys who came out to midfield as captains were transfers), and I wanted to see what the Final 4 teams' composition looks like.

This article really captures the story well (without mentioning Oregon).

https://www.saturdaydownsouth.com/sec-football/4-college-football-playof...

It's interesting that Texas, ND, PSU and OSU all have followed the same basic practice. They all are built primarily on their own recruits. They all added a smaller number of impact transfers. And each school has had major productivity from the transfers they took. Oh, all of them also lost many more players to the portal than they took. 

In a different article, I saw that for Texas vs. OSU, 19 of the 20 offensive and defensive linemen expected to play the most minutes in the game are recruits, rather than transfers.

Beyond Oregon, the article points to Mississippi and Florida State as programs that take massive numbers of transfers. And of course Clemson which famously didn't take them at all, though that is now changing. 

Bottom line. it is really hard - maybe impossible - to build a championship team by transferring in half or more of your starters. Championship teams will have good recruitment and development, and they will augment with around 6 impact players from the portal at positions of need.