I agree with you that these are rarely apples to apples comparisons, and that income vs principle stipulations can make it hard to see what’s going on unless you happen to sit on the investment committee (although most schools try not to spend down principle.) My point was simply that investments reported in the broad category of restricted are largely available for use.
My experience with non-profits has been that they have done exactly what @cinnamon1212 notes - they have gone back to the donor family to tweak the purpose of the gift to keep it from getting stranded. (Reluctantly and with a cost.) But my experience doesn’t extend to all 50 states!