Well I see the logic of what you are saying, the A320 system has the same layout in terms of hydraulic power split and reaction (half movement speed etc.) to failure. In theory what you say would work, in practice however realising you have a problem, working out what has gone wrong, working out what to do about as well as trying to make the aircraft fly and keep it flying is going to be impossible in most cases. Sods law means that it will happen on a short runway in foul weather etc. etc.
The fact is that there are some failures on aeroplanes that are more often than not going to result in a fatal accident despite the best efforts of the crew. The certification process recognises that fact and should ensure that failures of that nature occur very infrequently.