But these post-tenure 49% positions are the new thing to encourage people to go -- the previous option was emeritus, not only 0% but also unable to be the PI on grants etc. So they must have found a magic money tree somewhere to pay for them.

Not mentioned yet is the change from defined benefit pensions. If you had a final-salary pension, then there was no financial incentive not to go emeritus, in the good old days. Whereas on a defined-contribution scheme, retiring a year earlier does literally cost you (or your estate) a year's salary.

Large and famous universities sit on very large piles of money. They just choose to invest them differently — or maybe the monies are granted on particular conditions to spend them.

I mean a school like MIT must be able to afford opening more tenured spots, right?

You do get some growth by increasing the number of students you have and if research budgets expand but I'm guessing that is a function of inflation and population growth. Technology also allows you to teach more students with less faculty input so maybe you get productivity gains there.