There's a simple way around that, too:  use the fuel-tax money to rebate Social Security taxes on the first $X of income.  As fuel taxes decline, so does the rebate.
A much better idea, speaking as one who recently started an early draw of SS.  Those who don't want to plan on SS are welcome not to do so, but there are a lot of voters who have a certain attachment to the idea and a great many of us are not Republicans.