Well I have a few buts to that.
If, say TABCORB, decided not to take NZ or offer betting on it, I'm sure other operators would quickly take up the slack. As I mentioned, the Entain subsidaries almost for certain. They would also probably try to attract those customers to their NZ brands. Don't see how it would possibly reduce revenue to NZ TAB or NZ racing. Only to TABCORP.
The BIUCs are enshrined in legislation and set by the Minister, albeit in consultation including with offshore operators, but they don't really have much negotiating power.
On servicing smaller clubs, what you suggest seems highly unlikely to me if it even is possible under the terms of Entain's contract. It certainly wouldn't fit with their ethos as I sense it at this point. They clearly have plans to re-energise trackside, including a third channel and maybe more.