The linear ramp theory has been rejected by most modern archaeologists and engineers.
The ramp would need to be more than 1.5km long to stay at the gradient needed to roll the blocks uphill. This would make the ramp bigger than the pyramid itself by volume. It would have been continuously rebuilt to adjust the height, which would have made the process extremely time-consuming. 
The ramp would have been huge, but there is no evidence for such a large ramp. There is also a quarry and two cemeteries blocking three of the pyramid sides.
Although simple ramps may have been used at some sites in Egypt, they were only used for small pyramids. By some estimates, an earthwork ramp is only structurally stable at a height of up to 120 meters. It would need extensive scaffolding or stone blocks to support it in order to reach the heights needed to build the pyramids.
There are many problems with the linear ramp model. It would have been impractically long and structurally unstable. There is little evidence for such a long ramp, and no space to build one.