Why would you extend my branches if by invalidating them you would earn more coins?
I don't see why "invalidating" a branch/TDAG would enable you to earn more coins. That sounds equivalent to a double spend attack. But perhaps I'm not following you.
If 50% of coins will be generated via subsidy then every time I invalidate your branches by making them reference doublespendings. Invalidated branches = not spent subsidy.
I'm afraid I still don't understand your point. Perhaps I'm being stupid.
A broadcast transaction (be it standard of PoW) will not be accepted by the network if its ancestors contain a double spend. So any transaction fees the new transaction purported to claim will in fact remain unclaimed.
In this add-on proposal, I am not proposing that 50% of coins are generated via subsidy; rather, I am proposing that 100% of coins are generated via PoW.