It has been too long since I was reasonably familiar with the public transport networks in Brisbane, so some (if not all) of this is just me talking out my hat, but my impression of the light rail proposals in the late nineties and the network of busways that has subsequently been constructed is that, in part, they are serving different transport markets.
The light rail routes were more to upgrade specific routes that had reasonably high traffic levels within the route - the logic being that light rail was (perhaps - they were also seen as a means for urban renewal, rather than as a means for transport) a more efficient manner of delivering a certain service capacity along that route. Consistent with that, most of the proposed light rail routes that I've seen were fairly local to the CBD and inner suburbs in nature.
My perception of the busways is that they perform a role that is more similar to an express train - not so much driven by traffic within the route, but more by the need to get passengers (or perhaps just buses, given the way things were set up) from beyond one end of the route to the other end of the route. Consistent with that, the busways tend to or are planned to (there are exceptions) end way out in the burbs, and some of the buses then go and play on the normal road network.
There's considerable (if not complete) overlap between the markets and between the technologies that you can use to serve them effectively - it's not black and white, one is not necessarily better than the other, often modes are complementary, the distinction between "heaviness" of rail is rather arbitrary, etc. I just want to point out a distinction in role between the busway and light rail proposals that you mention.
If you keep that distinction and increase capacity further, then I think light rail becomes superseded by the short stop metro style train service, and busways morph into long stop heavy rail with feeder buses to the relatively distant stations (which is what others in this thread have suggested - but I don't think the context is right here). If you said "spend 7 billion on heavy rail then things might be different" I'd be more inclined to agree with you, but realistically $7 billion buys squat amounts of new build heavy rail in a topographically challenging, reasonably dense urban environment without any pre-reserved corridors.