In fact, that's roughly what happens. Click here to see the output. Sensitivity becomes a concern at large diameters. At this point, design becomes a little more intricate for the thin glass that we assume. However, studying the error as diameter is varied leads to a new surprise.
Error grows much faster than the square of diameter, as was previously the case.
The problem is that at this scale, the sagitta becomes significant compared to the thickness. We need to adjust the support radii as the mirror increases in diameter. If this is done, the radii and error are given by the plot below. The bottom plot is inner radius, next is outer radius, top is error. This shows that you can go beyond 900 mm, provided that you adjust the support radii.