New and different doesn't necessarily mean "better", and "better" should be our main focus. If using rectangular wheels rather then round ones would become the latest trend, would you switch your car wheels? No, because the round ones are *clearly* better in many respects. Look at how your thumb is constructed - it's much more comfortable to manipulate it in 3D space rather then to push and pull with it - the altitude of your thumb changes depending on its position because it's mounted on a semi-sphere joint. What else is based on a joint that's shaped like half-a-sphere? Oh, yeah - analog sticks.
Sliders are a good idea when there are certain space constraints and the analog input has to come from a flat device. On a full-scale controller though I just don't see a point.