Many creators are discovering that downrezzing 4K footage to 1080p for example, will yield better image quality than if shot with native 1080p camera. Especially in terms of overall sharpness, dynamic range and detail. For what I have seen, I concur, with a pinch of salt.
Before we throw our 5D Mark II and expensive lens collection to dust bin, I think there are couple of things to consider.
- Overhead of conversion of 4K files, practical aspect of editing
- Optical quality of the lens
- Available lens selection
- Quality of the sensor of the camera
- Camera rig / audio / etc.
If all these are taken into account and the workflow is practical, why not? However, I still think nothing beats optical qualities of the lens, mainly aperture size, bokeh quality etc. in DSLR filmmaking.