Oh, to be clear, I'm all in favour of hydroelectricity. I grew up beside a hydro dam with many neighbours who worked there. It's great stuff.
My concern is that we'd be talking about perhaps doubling the variable output needed from the existing system if we turn off the gas plants. Instead of needing 1 MW from them on a hot summer day, we suddenly need to ask them to provide 2 MW. Maybe it's doable, but it's asking a lot.
Imagine you've got a proposed project that will double the peak demand on my database. I say "What's the problem? The database works fine. There's no need to upgrade the hardware!". Same idea.
And climate change just adds more uncertainty, because droughts and floods are more common- while hydro does best with a steady input of water. I'm certain Quebec will be fine, because it's not planning to try to double the peak load on its plants.