A theoretical buying back of the western portion of 407 would be done in conjunction with construction of the 413 (GTA West route) that begins at the 401-407 interchange. The rationale is to create a more unified and continuous road network in the western GTA, particularly for those commuting into or away from western Mississauga and into Brampton. The 413 would then have direct N-S access to QEW, 403 and the 401 before traversing north through western Brampton towards the 410 and 400.
Buying back the western 407 through Halton allows for a more direct connection to Highway 403, providing a seamless route from Burlington to the 410. The rationale here is to provide an adequate E-W alternative to the QEW which will only get worse traffic-wise in the coming decades. After the HOV lanes were added, there isn't much room to add an additional lane. The N-S 403 through Mississauga is also a pretty big bottleneck, partly because of the awkward 403-407 interchange.
Again, my rationale is to optimize the road network and prepare for an influx of people into the Western GTA and Halton over the coming decades. The aim is not to lower operating costs for 407 users in those sections; rather this is a pretty underused segment of highway that perhaps would be better off in provincial hands and integrated into the existing highway network in a more meaningful way. It would seemingly be easier to buy back this section of 407 than the heavily traveled central segment. I'm not familiar with travel data on this portion however.
