A good way to exclude old corrosion-riddled GA aeroplanes from the 750 kg rule would be to make it valid only for aircraft designed or first flown after a certain date (say 1990?). Existing RA aircraft designs exempted or 'grandfathered' somehow.
Going to 750 kg will allow more robust designs, and that can only be good for RA safety. These aircraft will still have a stall speed limit of 45 knots, so we are not about to see any space-shuttle stuff ripping around frightening the horses.
A lot of existing RA aircraft are really lightly constructed, so it will be interesting to see how well they stand up over a 20 year life-span.