I think I agree. There's some grumbling that proposed features don't belong on OSM since they are not (yet) physically there. For example, see https://wiki.openstreetmap.org/wiki/Key:proposed and the discussion page - which still has no consensus.

 But in this case, it is a major project, and clearly verifiable source-wise, so that's not the real issue. Being a major and probably THE most major proposed line of Project Connect, with the way it will go to the airport, it will almost certainly be built, even if a ways out (and more expensive). So on that, I personally find a lot of value in seeing the outlines of future roads and railways - I find it useful information. If others agree (and judging by the way Austin as a local community often maps proposed features like this, I suspect enough agree to make this make sense), then leaving it is fine. 

So, I'm good to resolve this, for now.