Metrolinx has to be involved with GTAA somehow, as Metrolinx is going to run a lot of services to that transit hub. Metrolinx won't run services if GTAA builds it somewhere that Metrolinx can't afford to get to. So Metrolinx is involved, because they control where they build the rails.^ I think that opportunity is simply gone, unless the GTAA and Metrolinx play Sim City like China or Dubai. It's simply not politically justifiable to rip up a spur which is still brand new. But it's definitely within GTAA's interest to reduce congestion on surrounding roads and thus improve transit options beyond a dinky bus stop in the basement.
GTAA and Ontario(Metrolinx) definitely needs to team up, in order to pull off such a project. There's no way to do a good Pearson Rail Hub (or two) without pulling in a lot of Metrolinx services (whether it be Eglinton Crosstown, GO trains, GO bus, UPX, RER, and probably HSR). They can't afford to let high speed trains go by (and take business away from Pearson) without building a good Pearson integration to compensate for the cannibalization (e.g. Toronto-Ottawa flight traffic may be displaced by HSR). It's in GTAA long term 30-year interest to make sure that they are involved in a Pearson Hub, even if an Ontario company (Metrolinx) ends up the one funding the building of the Hub.But what *does* it have to gain by hosting a transit hub on its own land? The hub will not generate by itself any revenue (aside from a convenience store on the platform). As long as travel times are reasonable, YYZ has no need to become a (ground) transit hub by itself. It has a near-monopoly of travel options of 8 million people.