Ah yes, let's set fire to things to combat global warming 🤔
Trees are regenerating and growing well on Dartmoor wherever grazing (especially with sheep) has been restricted. You can see them marching up the slopes of Yar Tor, the sides of the River Dart Valley, etc etc etc. Vixen Tor is a good example- it's been fenced off for years due to a long running dispute, and within the fence lots of small trees are growing happily.
Lustleigh Cleave is another interesting example- it's now woodland, but until the 1960s it was classic bare moorland. Lustleigh is quite a posh village though, and the people who inherited the commoners' rights had no interest in grazing it, and the woods regenerated naturally. I'd say the natural tree line would probably encompass most of Dartmoor. But they could do with getting rid of all the fucking conifer plantations, which are no good for any ecosystem (and quite flammable too, I would have thought 😬).
But no, unbroken woodland wouldn't be the ideal for the whole of Dartmoor, but neither is the current state of the high moors- basically a bare desert, with very little bird life at all. (Swaling doesn't- or shouldn't- happen on the high moor). Less sheep (and plenty of ponies!) would hopefully create more of an open, mosaic environment.