Simple elementary arithmetic (at least for Sal Kahn and Bill Gates) …
For various reasons including demographic shifts, an increasing number and percentage of the US population will be dependent on “government” for an increasing amount and percentage of their personal welfare. And, we don’t mean citizens and non-citizens already on some sort of official govt welfare program.
Common examples are the aging of America and retiring Baby Boomers.
In general, government will be called on by more and more individuals and organizations (for profit and non-profit) to do more and more with less and less tax revenue.
There will be more Takers taking more, and fewer Makers who will be making less.