That in fact has been suggested, either have a spoil board that you can take out or drop for thicker materials or raise the material so the lever action of a long Z axis is minimized for most of use. The ability to drop the bed seems a better option because you aren’t dealing with long corner tubes which could introduce flex.
Here’s how I have been thinking about it.
Three of many considerations for establishing z height: 1. If you are doubling as a printer too, folks want more Z, but that compromises rigidity for milling. 2. To work on thicker piece of material, but since endmills are going only 20mm depth before collet interference, that does mean that you aren’t going to be cutting material thicker than that in normal cases. 3. Carving the surface of thicker material, like a cutting board. In that case, it would be better to have a drop center with a removable spoil board since you aren’t going all the way through.