Sydney has long needed something like Westconnex. Here's what the motorway network looks like:
https://roadsaustralia.weebly.com/uploads/7/7/9/8/77988430/s...
The peninsula to the east where the M1 is the city core and everything around it is very, very densely built up. Before WestConnex, from where I live in the Inner West, getting to the airport, under 10 km away, required zigzagging through residential neighborhoods and going through approximately a million traffic lights. There were no motorways to the city, no motorways away from the city, and no motorways radially around the city either.
WestConnex, the bit in blue, fills all three of these gaps, and since it's all underground it's out of sight and didn't require demolishing entire neighborhoods like previous 1970s-era plans did. And having driven through the newly opened interchange yesterday, I don't really buy all the "confusing" whining either: follow the signs and it'll get you where you want to go, you don't need to worry about 15 other tunnels going in other directions.
The usual argument against building more roads is that they just induce demand and immediately jam up again. This is why I think the high tolls are a feature, not a bug: they ensure people will only use the roads when the time savings pays off. (Although I do wish Sydney would bring in Singapore-style dynamic road pricing instead of flat fare that means the tunnels are sitting around empty on Sundays.)
What you can criticize WestConnex for is the corruption and the opportunity cost: that $18B could've bought a lot of other things too. But on the upside, Sydney is finally getting serious about metro and light rail, so things are looking up on that front too.