I suppose the question at the back of mind is this.
If Sydney is forecast CAVOK, should you carry an alternate? I'm looking out the window at the moment at a Botany Bay and airport skyline that only has single Chemtrail ( ) to mark the sky. I'd plan to arrive in Sydney on a day like today with about 75 minutes worth of fuel. You want me to turn up with more than double that in case of.........
If I'm flying from MEL-SYD I have CBR from about 10'000' on descent. If CBR's gone u/s due FG in the morning but Sydney's wide open you think I should have a return to Melbourne? My 'normal' fuel means I have Richmond anyway but that's considered an 'emergency' field for my operation
From an EASA perspective, that's the whole planned v enroute decision making spektrum.
No, there'd be no reason to return to Melbourne- you'd land there with the same fuel as Sydney, so where is the safety advantage in that?
The thing would be, you departed with fuel to reach two different places should one become unusable- in your scenario, one did.
The fact that Sydney has two independent runways is relevant- but lets take a scenario like Perth. it's CAVOK so you ONLY have fuel for Perth when, as number two, another aircraft becomes disabled on the intersection- NOW things get interesting.
Now, before the peanut gallery gets excited, you WOULD be allowed to commit to Perth with no alternate using en route commit rules- but you would have to DEPART with planned fuel to an alternate.
The chances of having an airport go unusable on the rare occasion you have to commit are obviously much lower than if you depart with nothing but fuel for destination on a routine basis.