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.