I don't know about expanded GO train service, serving the needs of Humber Bay just fine. Not everyone is heading to Union or the surrounding area as their destination point, and let's remember that the existing GO fare structure acts as a detriment/barrier to many.
Of course there have been musings of a restructured GO fare system, but with the current provincial government in power don't hold your breath on that ever happening. Heck, if another provincial party replaces them, I still wouldn't even hold my breath on the current status quo changing either.
As you mentioned, the Waterfront West LRT has been in the planning stages for more than 30 years now, and that's where it's probably going to stay for the next 30 years minimum knowing how this city operates. As it currently stands, the only concrete transit improvement in the books are a Park Lawn GO station with ~30min frequency. Much better than nothing, but certainly not enough for an area that's going to have densest population in Canada when all is said and done.