Large Animals, Little Food, happened before...
The indricothere, of the Rhino family, managed to survive for a few cool million eating from rare trees in scrubbrush land, dealing with wet and dry seasons much like the current african savannah (only without the grass).
However, there are enough grass-eaters out there that I don't think evolution's going to produce new tree-eaters except ones that eat fruit as part of a omnivorous diet (like primates today).
Nature's produced grass-eaters (including ourselves, though we can only deal with cooking the seeds -- wheat and barley are grasses) in quite a few mammal and lizard & salamander families, and I don't see the grass-eaters giving up and tree leaves being the core of the future diet again. Tree leaves are a luxury item, like giving carrots and/or radishes to a rabbit, but more mammals still need a steady regimen of grains.