You can always use a crop as a backup, or to close extra distance between you and your subject without resorting to a more expensive and heavier supertelephoto.
I do this myself, but I don't make my entire living from images, just a small portion. I do shoot with people who go full time and they absolutely trash crops any chance they get. The reason is noise. They've been burned too many times with what they thought were going to be great images at ISO 800 and 1600. The noise and IQ destroys fur and feather detail for larger prints.
All the AF focus points in the world and FPS won't make up for a lack of shutter speed or too much noise. Get the sensor
right first, then start piling on the croutons and dressing.
It just doesn't make sense, IMHO, to spend almost $2000 on an inferior sensor. The 70D is a solid camera for the money, and about the limit of what you should pay for APS-C image quality.