The $2000 number is a cap for wealth, not income.
The problem isn’t having a phaseout or not anyway - we could set a wealth cliff of $50k and that would be perfectly fine. It’s the numbers that matter, where the phaseout or cliff is placed and how steep a phaseout. $85 a month is a redonkulous place to put the phaseout even if phaseouts are “better” - that is a worse policy than a 50k a month cliff!
And you’re right, the reasons the number hasn’t been adjusted since 1935 (in 1989 the number was inherited from a prior program, it wasn’t adjusted at that time) is because of political will. Social spending on anyone other than the elderly is extremely unpopular in the US.