I wouldn't be so quick to write off this project.
The location and configuration of the busway is comparable to the Northern Busway in Auckland, which has been phenomenally successful, to the point that plans for a second harbour crossing (technically, a third crossing) have been put off because the traffic volumes over the Harbour Bridge have reduced significantly since the Busway opened.
Much like what is proposed for the Doncaster Busway, the Northern Busway runs for just over 6km beside the Northern Motorway, from Constellation Dr to Esmonde Rd, where buses re-join the Northern Motorway for a further 6km across the Harbour Bridge and through the traffic funnel that is Fanshawe St (much like Hoddle St is in Melbourne) on the way into the CBD.
I think the success of the project in Auckland is that, at least in part, the Northern Busway operates like a railway or tram service, not just as a bus lane. The Northern Express service along the Busway uses a dedicated fleet of double-decker buses, and only stops at the designated busway and park-and-ride stations that local bus routes feed in to. These stations have facilities comparable to (and in some cases, much better than) most railway stations.
In regards to timetabling, NEX buses run at 3 minute frequencies during the morning and afternoon peak, reducing to 10 minutes for most of the rest of the day... which is comparable or better than most train and tram services. In terms of journey times, the trip from Albany Station (about 4km beyond the end of the Busway) takes a bit over 30 minutes, which is way
faster than you can do the trip in a car during the peak.
If PTV treat services on the Doncaster Busway in a similar way (instead of using the Busway as a general-purpose bus lane) I don't see why this project couldn't be equally as successful.