Every single person that has SP today (no matter what the amount or when it was acquired) earned/bought that SP under the premise that the SP would give them influence over the site's rewards pool. There are people that invested heavily early on specifically so they would get that power. If the rules were changed such that your SP only gave you power over 75% of the reward pool - then it is violating the agreement with everyone who currently has SP.
Imagine you were founding a startup company, and you got 5 venture capitalists to invest in your company. You formed an agreement with each of them that they would provide services/funds in exchange for 10% control of the company (each). Then a year later after the company started to grow, you decided to say well, actually - we are only going to give you each 7.5% control now. We want to take the other 2.5% and give it to our employees, so that they have a say in the way things are done as well. That would be a violation of the contract that you formed with the original shareholders.
[Edit] Updated the numbers of the example to better line up with the percentages in your proposal.
Thats exactly why I have bought and earned SP and I still have no real influence or any real hope of getting any under the current system.