What effect will the halt of the use of cpu-gpu miners have in the difficulty? Do u have an estimate about what will happen when more powerful machines make the simple gpu use unaffordable?

This would practically mean that significant part of the mining will halt. You think that the new machines' impact on difficulty will be reduced by this or this phenomenon will be not an important factor?
It won't matter. The difficulty will rise until the most efficient miners can't make a significant profit. What inefficient miners do has no effect because their actions are balanced out by efficient miners.

Essentially, take the most efficient mining setup widely available and compute the difficulty at which it just breaks even. Market forces will, over time, push the difficulty to be a bit less than that. If the difficulty is more than that, efficient miners will leave, reducing the difficulty. If the difficulty is less than that, efficient miners will enter, raising it.