I could make exactly the opposite argument. Namely that you won't attract investors effectively if something is too inflexibly defined before the long term effects of that rigid decision are understood. Sure BTC is doing okay, but really nobody knows what the hell is going to happen when the rewards run out. In fact every somewhat credible analysis I've seen suggests it will be bad.
As an investor, I'd rather see some flexibility retained until a system that is clearly workable is invented, rigorously scrutinized, and well-tested in the real world. That hasn't happened yet.
If you really want a 100% fixed or 100% locked-in supply then this coin is not for you.