Why would you extend my branches if by invalidating them you would earn more coins?
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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.