That's just conflating issues. Miles or no miles reward system, customers should always have the option of withdrawing at will, barring technical difficulties.
Imagine if a bank were to restrict withdrawal just because a bunch of customers decided to leave due to them lowering interest rates. Where got such thing one?
Not being able to withdraw indicates a much larger and deeper issue, IMO. That the money is tied up in some other things that are not so liquid and potentially high risk. It also potentially indicates that they have very low liquidity buffer to begin with.
I find it rather hard to believe that a bunch of customers withdrawing can lead to this, unless it is the overwhelming majority of their customers doing so. In which case, the pertinent question should be whether cancelling the rewards programme is a good idea since the survival of its business practically hinges entirely on it drawing in and keeping customers? If miles reward is your only gimmick, then of course people will start leaving in droves when you remove it?
It's also disingenuous of them to blame it on "gaming" when they knew from the start that there was such a possibility and they very well accepted it as a risk of doing business.