Converting the Williamstown and Altona rail lines is not an economical saving decision, otherwise, the Upfield line would have trams running on it. But that idea was ditched.
Why does no-one talk about converting the Alemain line (that's a quiet line)?
Melbourne's west has the poorest railway infrastructure, considering its growth.
In the other growth corridors
The North got Craigieburn electrification 2007, a new station at Coolaroo in 2010, Sunbury electrification in 2012, South Morang in 2012, Mernda in 2018.
The Southeast got Cranbourne electrification in the 1990s, Cardiner road station in 2012, introduced 10-minute services or better from Dandenong in 2012, Cranbourne line is getting a full duplication, new signaling/HCMTs underway, but their triplication project got canceled & will be the first to receive the orbital railway.
The west gets Regional Rail Link in 2015, little improvement to timetable on Werribee line after RRL opened, Duplication to Melton is opening, but all of those are regional services, a new station at Williams Landing (which is so busy, that it makes sense to keep Aircraft as a back-up), and western rail plan is only a plan. Metro 2 tunnel is delayed, which is what the west needs.
And now economically saving in someone's mind is degrading the service to light-rail which has less capacity than the existing service. The Williamstown/Altona corridors are to serve the suburbs of Altona, Altona North, Newport, Williamstown, Spotswood (potential development), Yarraville, Seddon, and South Kensington. Werribee line will express past the inner-west via Metro 2.
The inner-west needs to be developed more and this has been happening, have you been to Altona? High density exists along Pier street and there's potential for more development in the future. Altona line could always be used as an opportunity to be a branch line to Point Cook. This is long term thinking, short term thinking may involve light-rail conversions, but there is more to the west than urban sprawl.
If you upgraded Melbourne's western rail lines, I'm sure the west would have more patronage and more residential growth going upwards. At the moment, the west has a poor-service for transit and because of this, we're building a west-gate road tunnel instead of metro 2.