How long before boosters decrease their donations and just give it directly to the players?
Immediately, and this is what I've been saying about the two main arguments as to why this won't be that big a deal.
Claim 1. It's not not like boosters have been withholding money waiting for this.
Response: Correct, but before a booster gives $500,000 gift to an Athletic Department that goes for operations, buildings, etc. that have some effect on steering recruits, but in the current age of everybody having good facilities, that impact is marginal. Now that booster shifts that $500k toward likeness income to get players to their school.
Claim 2. It's happening anyway, might as well make it legal. Yes, some of this stuff happens, Response: but people WAY overblow how much money is getting to players and handlers under the table. Look at some of the rumored $ amounts on the top end to some of the biggest HS basketball recruits in recent years. It's clear that the threat of sanctions kept these in check to a large extent.
Claim 3. The same teams go to the playoffs, so what's it really matter.
Response: Just by reducing scholarship limits the past couple times it increased parity in CFB significantly. Think about that, when every team had a lot more scholarships, 3 star type guys would go to Oklahoma instead of Iowa State, and that was in a scenario of financial parity between the two. Now imagine that same scenario where there's an additional financial benefit. Look at how the Dakota Schools moved up from DII to FCS and started kicking everybody's ass, including former power UNI. The Dakota schools have enough money to pay actual cost of attendance, so what probably adds up to an extra $3-4k per year.