I'm sure that if it was simply a matter of using the same infrastructure and adjusting the scheduling to accommodate both the high-speed and regular trains then we would have had high-speed rail decades ago.
I make no claims to knowing anything about high-speed rail, but I do recall issues such as the technical specs of the tracks to handle the higher speeds, turn radius limitations, road crossings, electrification vs diesel, etc. all being issues to be resolved.
I don't know enough about the Alto plan in particular to express an opinion on it but while I'm in favour in principle of a high-speed rail link joining the major cities in the Toronto-Quebec corridor as an alternative to air travel it all comes down to the details. What will the total cost end up being? What's the share of that cost between the public and the private partners? What is the expected economic benefit of the link? What will be the cost to use it compared to regular rail or air travel? Is there a plan to allow the Government to recoup the initial investment cost over time...how long? Etc.