I suppose I should clarify when I say suburbs I mean anything that has absolutely zero density. Streetcar suburbs have the building blocks to support intensification better.
But what about North York? They have the highest per-capita subway ridership outside of Downtown Toronto. What about the terminal areas of the Bloor, Danforth, and (previously) the Spadina Subway? Don't these areas have overwhelmingly excellent (or at least decent) ridership to support the services they currently have?
Sure, RER is an extremely important piece of a missing transit network in the city, but it exists to mainly serve separate suburban cities (Pickering, Oshawa, Oakville, Aurora, Brampton, Mississauga, Markham), all of which are much farther than the current scope of the existing subway system, whereas the subway extensions to the STC or RHC exist mainly to serve people in the city of Toronto, or an area directly next to it. You can certainly have both in a healthy system, and oftentimes for commuters, having both increases overall transit usage astronomically because it gives people choice. RER studies have shown that RER will only remove, at best, 10% of customers from the Danforth subway, which is insignificant over the next 20 years, this is largely because RER would be more crowded than most of their subway commutes, travel times would be the same between North York/Richmond Hill/Scarborough and Downtown on either, and the fact that RER is less flexible in its destinations. This is not to say that it is bad, but that it serves a much different market.
But I digress, we're talking about the Ontario line here, and even the density of a streetcar suburb is not really enough to truly support a subway. The relief Line/Ontario line is mainly needed to funnel suburban commuters from Scarborough and North York off the Yonge line and away from Bloor Yonge Station, all while increasing redundancy in the network. The streetcar suburb's presence only improves the business case for this line (in the form of increased property values, options for densification, economic stimulus, improved commutes for those that rely on the 501/502/503/506, and added ridership). This is excellent for the project but isn't the ultimate goal of it.
Don't get me wrong, South East York and the East area of Old Toronto certainly need better rapid transit, but if the Yonge line didn't extend past Eglinton and the Danforth line didn't extend past Main St, the Relief line would not be needed.