Neat idea!
I don't like the economics of it, though.
First, it reinforces the "if you started mining early you are rich only because you were lucky enough to be early" idea. And a lot of people already think that is unfair; give the early miners a current mining discount and you're just "helping the rich get richer."
Second, what stops an attacker from offering early miners $$$ for their (already spent) coinbase private keys? If they have value because they give a mining discount, then there WILL be a market for them. A wealthy attacker could just buy up as many as they can find and then take over the network with less hashing power...