I know I'm beating a dead horse, but when people say "[organization] has no plans to do [idea]" to discredit an idea, it shows a lack of understanding a lack of understanding of good planning. Planning timeframes are limited to a few decades, at best, and, for better or worse, are highly influenced by politics. Short and medium term planning, which is what people are referring to when they say "there is no plan to do this", is different from long term contingency planning. I think Metrolinx does good work, but we need to understand what their processes are actually like. They are a short- and medium-term focussed organization. That's their job.
And sometimes, even medium-term is questionable due to those aforementioned political influences. If you asked Metrolinx two years ago whether the RER concept (as currently described) was planned, they would have said no ("but we're considering it and it needs further study"). If you asked Metrolinx five years ago if electrification was planned, they would have said no ("but we're considering it and it needs further study").
It's impossible to know what the future will look like. "Prediction is very difficult, especially about the future." But that doesn't mean it's not necessary or worthwhile.
So how does this relate to the Davenport Diamond issue? Because even though a third or fourth track aren't planned at this time, there's still good reason to believe that they might be needed in the future. Why? Because that's how every single rail corridor in Toronto, in Canada, in the world evolves. Increased demand -> need for more capacity, such as more tracks being needed. High demand for longer-distance trips (such as Barrie) -> need for more tracks to separate express and local trains. This is what happened on Lakeshore over the past few decades. This is what happened on the Georgetown corridor over the past few years. With the level of planned development around the Barrie corridor (for one, consider the land that is within the growth boundary), it's not unrealistic to think that this WILL happen to the Barrie corridor in the future.
Look, it's fine to say that something isn't planned at this point in time. But that doesn't mean we shouldn't consider longer term needs or that it's OK to plan things in such as way that precludes potential expansion.