> Benefits of this proposal outweigh the cons. Average length of patent protection will decrease. reply

Why not do that in the straightforward way of simply decreasing the patent protection duration? Or as other people have suggestion: make it contingent on the type of thing being patented.

Because different patents might have vastly different value. If someone has invested a billion dollars to develop some new technology, they might want to benefit from it longer than someone who invested $10k. Yet, this scheme still allows the $10k investor to keep protecting his invention in case it's wildly successful.