Once you dispatch you make the best decision you can with the available information and time available.
The end
Not strictly true GG.
When you pass your EEP (EDTO Entry Point) it is as you say. Up until that point, however, if the weather deteriorated at your island divert - or you had a mechanical issue rendering you no longer EDTO compliant - you'd have to turn back.
It just so happens that many Australian airlines have chosen to pervert this regulation and declare their EEP as dispatch, not at 90 minutes out from the last suitable aerodrome which by strict definition is the beginning of the EDTO /ETOPS area.
The obvious commercial advantage is that if the weather is good when you takeoff at all your bolt holes across the Pacific then you can sail on your merry way to HNL, even if every one fogged in 2 mins later. Other airlines would reassess the bolt holes just prior to their EEP which might be halfway into the flight and if they have deteriorated below EDTO Alternate minima, then the flight can't continue on that basis.