If the Richmond Hill line were straight and fast, you'd have a point. It isn't though, it's long and very windy down the Don River - at one point coming within 4,000 feet of the Victoria Park border of Scarborough!
Travel time on GO from Langstaff to Union is currently 42 minutes in rush hour (13 minutes to Oriole near Leslie TTC; 29 minutes from Oriole to Union) - and there's only one station in between!. If they build RER it will likely come with additional stations, so MU operation won't save much.
Finch to Union on subway (at least before they started messing with the signals) is only 28 minutes. How much time will those additional 6.4 km ( 5 stations) add? Well, Finch to Lawrence is 4 stations and 6.3 km and 9 minutes. So add a minute of dwell time ...
Who is going to take an RER train that only comes every 15 minutes and takes 42 minutes to Union, when the subway comes every 2-3 minutes and takes 38 minutes?
Other than a handful who actually work very near Union Station or perhaps on Queens Quay - very few.
Might as well stop the GO trains at Oriole/Leslie or even extend the DRL further up the tracks to Steeles or into Markham at Yonge and Highway 7. Then run GO RER from there.
To add to this, under the present alignment, an interchange with the Bloor-Danforth line is impossible, and an interchange with Eglinton Crosstown is highly impractical. Oriole GO would need to be relocated in order to allow for an interchange with the Sheppard line (at least this could be easily done). It is unlikely that a Relief Line station would be built in between Lawrence and York Mills to allow for an interchange with Richmond Hill. This makes the Richmond Hill GO line a zero for four when it comes to interchanges with the TTC subway system. In addition, any move to RER on Richmond Hill would require electrification, which can't be done with an alignment that goes through a floodplain.
If we wanted to address these problems with the Richmond Hill GO line, and upgrade to RER, then it would necessitate a new alignment on major portions of the corridor south of Lawrence. This new alignment (alongside the new stations) would cost a lot of money. All to duplicate rapid transit service in the same general corridor of the city as the Relief Line.
You already detailed how travel time from Langstaff to downtown is not considerably improved on Richmond Hill GO compared to the theoretical TTC subway extension to Richmond Hill. This is in addition to Richmond Hill GO servicing only one destination (Union Station) when many people have destinations at Sheppard, Eglinton, St Clair, Bloor, and Dundas. An RER train arrives every 15 minutes, whereas a subway arrives every 2-3 minutes, as mentioned. Not to mention that it is a higher fare price than the TTC.
Now let's consider ridership. The Richmond Hill GO line has 10,000 daily riders. This is the lowest ridership in the GO Rail system and a fraction of the other GO lines, or 1/6th of the Scarborough Subway. If the Richmond Hill GO line was a TTC bus route, it would be the ridership equivalent of the 11 Bayview bus (10,400 daily riders). There are bus routes in York Region with more users, and York Region has overall very low bus ridership.
So I understand the refrain from wanting to discontinue a GO Line, but given the myriad of problems with the route, the prohibitively expensive cost of upgrading the line, the low ridership, high fare, high operating cost to the GO line, the single-destination (Union Station), and the duplication of service on the same general corridor with the planned Relief Line, it needs to be emphasized that is not a particularly great GO route, presently or as a priority to upgrade.
I don't think it is ridiculous to suggest replacing the route south of Langstaff with the Relief Line. You would be replacing a route with all the aforementioned problems with a frequent, rapid, and direct subway line that connects with Sheppard, Eglinton, Danforth, East Harbour, and finally the Financial district (and one day, potentially destinations to the west).
