IMO things are moving towards larger, better financed operations in the ACMI world.
There's a growing demand for cargo-lift out there, but... At the same time, there are changes afoot that will increase the starting, and operating costs of driving freight across the sky...
RNP requirements forcing better avionics, various regulatory changes, and increasing staffing issues have and will pinch the bottom line. But IMO it will be increasing fuel costs and carbon taxing that are going to really alter the ACMI landscape.
The day might come (in the not so distant future) that many of the current crop of air-freighters are no longer profitable to operate. Not that cargo or ACMI flying will cease to exist, rather there will be a move into newer airframes such as the B777.
Sure, you will always have the fringe operations able to make a buck with classics on some niche markets, but that will be pretty cuttroat and like this thread implies, there will be a lot of comapnies that do not make it.