It's not really that it's a terrible idea, but I'm concerned that if they did set up a heavy rail route on existing ROW they would be cheap about it and the line would terminate in Strathcona requiring extra transfers to get downtown, as opposed to the LRT which is already connected. Even if the ride was longer at least it's no-nonsense direct to Downtown.
Unlike Toronto, or even Calgary we don't have a convenient station on a heavy rail line downtown that could be repurposed or adapted.
That said if they want to set up a dedicated express line direct from the airport to downtown, then giver.