There are 131 D1 schools. Assume 85 players per school. That is over 11,000 players not including non-scholarship players. Assuming more than 3000 hit the portal (the number last year), 2/3 of players are still on that team. Those 2/3 are still important, still need to be coached and developed. While the top 40-50 super stars in the portal are going to push a team to the next level and be fought over, the core of the team is still going to be there. The stats I've seen say about 30% of P5 transfers go to another P5. Most move down or are still sitting in the portal.
So, you still need to recruit and develop players. If you have a strong culture, if players trust their coaches, and if they are offered enough incentive, many will stay. But like any other job, you are going to lose some to the portal. You need to be able to leverage the next man up.