I know a guy has been doing tree work for 30 years. A few years ago he sold his bucket truck, a nice one with the bucket over the chip box and the boom was on an elevator, and used the money to buy a bobcat skid steer , and two excavators, a big one and a little one. He also has a custom built knuckle boom crane. He now probably makes as much doing digging and craning as he does trees. He sales and sets boulders too.
It has the added benefit of being able to use the equipment for tree work when needed. He has every attachment you can think of almost for his skid steer and excavators too.
Ive work for a few companys that also did demos and land clearance beside trees. They kind of go together.
I think if done right being a little diversified can keep you working in the lean and slow times. By that I mean things that might go hand in hand with tree work, cleanups, land clearance, hauling, hardscape, etc. I do weed abatement on the side my self, and use to do Code enforcement work for the city. Anything to pay the bills