The subway extension is supposed to cost well over $3 billion. That amount of money could buy you any frequency you want for an RER branch to Scarborough Town Centre, regardless of the current RER/Smarttrack plans.
$3 Billion would not get you anywhere near subway-like frequencies on an RER branch to STC. The upgrades necessary to enable that would be staggering. Rail corridors would need to be widened, grade separations would be necessary to eliminate conflict in train movements, Union Station would need to be massively reconfigured, likely with a tunnel running beneath it, signalling systems would probably need to be changed. This particular branch would probably need a new fleet of trains, more similar in size and design to the Toronto Rockets than the current GO fleet, because long, commuter-style trains aren't well suited for frequent subway-like service (because it takes them a significant amount of time to switch tracks).
I hate to poo poo the idea, but it's simply not realistic. We probably aren't ever going to see subway-like RER frequencies until we have an RER tunnel running through Downtown, in addition to a myriad of other auxiliary network upgrades.
That said, 15 minute frequencies don't eliminate a rail line from being rapid transit. Some
subways operate less frequently than that. Atlanta for example.
Toronto isn't Atlanta. There are different service expectations in the two cities, for various reasons.
When RER is done right there's very little to distinguish it from a subway or any other form of rapid transit.
An example of RER done right is London's Crossrail. Toronto RER doesn't cut it. It's an improvement, yes, but it doesn't hold a candle to London's implementation. People in this city already get annoyed when they have to wait five minutes for a train. The fact that RER trains come 1/6th as often as the subway network is something that will certainly be noticed.
None of this is to say RER is bad (it's not), but it's potential is being greatly overstated by people on this forum.