You're correct about losing resolution and image quality, and I should have mentioned it. I just hope that people don't think that they're going to have the magnification of a real 2000mm lens just because they have a smaller sensor in their camera.Nikon has made a 2000mm lens. It uses a mirror and is stuck at f11.
As for cropping-- the P900 uses a really small sensor with 16 MP. So you couldn't crop a D800 or Canon 5DR very far before you start losing theoretical pixels. OTOH, he practical resolution is probably rather constrained by diffraction effects. Might be useful for a birder with a budget.