If it was as easy as you say it is, it would've already happened. The problem with this (and the reason why Richmond Hill RER will likely never be a thing) really comes down to 2 things. 1) CP, 2) Interchange accessibility. The first issue is how do you get the Richmond Hill Line trains on to the CP Mainline. Even if we were to assume that the existing bridge is still in good condition and that physically getting onto the tracks is easy, you still have to deal with sharing tracks with CP for a 3+ km section, which if the Milton Line is anything to look at, wouldn't be the most convenient setup at running any form of. Best case scenerio you build the missing link, somehow manage to convince CP to use the York Sub (which is its own nightmare), and then you can have full access for the Richmond Hill Line, and even then you're still stuck with a few problems. First being these proposed interchanges. Sunnybrooke Park can work, Langstaff can work, Broadview though, uhm, good luck with that:
Finally then you have to grade separate Doncaster Diamond, and find a way to share tracks with CN north of said diamond which is certainly possible but altogether this project would be the most expensive RER project in the region, and what you get is at best an "okay" RER line. There would generally be a better cost to benefit ratio just to build another proper relief line, and do note I am saying this as someone whose closest GO line is the Richmond Hill line and would absolutely benefit from a Richmond Hill RER.
P.S. Metrolinx is planning to build a major GO RER MSF right at the foot of the bridge to the CP tracks, and plans to use the existing tracks to park their trains (although Via apparently wants to use that corridor for VIA HFR so this might be subject to change).