Again, very usfull stuff in there.

Am using motors as i can easly get the resloution i need and if i make a mistake i can change them reletivly cheeply,

Nor you can wear Hiwin bearings if properly mounted. When you have them in your hands you will see why. They will be probably that last element in a machine to fail.
You not considering the difference in wear when using stone, Its not just the heavy use on the machine parts, your basicaly air-rating grinding paste. allthough the powder feels fine and soft, on a particulate level, its rock hard and sharpe. vacuums and covers can pre-long bearing wear, but, you are going to wear rails and bearings when cutting or surfacing stone. Then think that large beds are hardly every fully utilised, everybody's got there own favourite sweet spot on there machine were most of the smaller jobs get done, so you get uneaven wear.

If you then want to surface a large bit of stone, 1500mm by 800mm, you would see the wear patterns in the surfacing as the rails and screws go between the commonly used area and the less used area.

There are other considerations as to why it might be a good idea to have the bearings as the failure points. Re=straightening a frame, or gantry, is quite a long and complex job, that involves building a jig, usualy into a thick concrete floor, and gooing through a heating,cooling and tighnening routines, then when its in shape, heat it and cool it to even the stresses.

8/10 times, it would be cheaper to just remake the gantry, and salvage the unbent parts.

whereas, if you specifically design your frame for the bearing to be the week point, servicing is easier, serviced periods are calculable and variable depending on machine use. and as long as your bearings will fail, beyond the maximum cutting forces your machine will exert, then you could consider it a safety feature.