It isn't an analogy, it is a fact. If a student athlete wants to transfer up, they are gambling that 1. a school in the P5 will offer them a scholarship and 2. that they will be able to start there. If you are a current starter, you will want to start somewhere else. Moving up to a P5, competition for those starting spots will be more intense. You have to bet on yourself that you can earn that spot over all the other guys that are competing for that spot. If you don't, you have made your situation worse and lost the bet.
How many G5 players leave to go to P5 currently? I don't think the number is that high, especially compared to the number of P5 players going to G5. I just don't see this narrowing competition. It may actually help us in that more of those players will transfer to a G5 earlier in their career instead of waiting until they don't have to wait a year to play. They may be able to see where things are for them and make a move they see as more advantageous for their future before they sit for three years.