If the regulator is happy to issue them with a licence to operate the aircraft and the insurance company is happy to cover them what case can be used to fight it. As far as I can see only a hypothetical threat to our T&C's. That is all Balpa has, and without, I suspect, a majority of the workforce's support.
You can try and build a case on safety but good luck with that, especially as any one of us could all be subject to an accident with a small amount of misfortune.
Three things will stop this:
1. A change of regulation (as per the FAA)
2. Public demand (most people don't care)
3. Supply / Demand rebalancing (most likely)
4. An accident (very unlikely not to mention unpalatable)
The union have very little power to stop it unless we all agree to walk out...