The Richmond Hill GO line needs to stay right where it is to bolster the case for electrifying the corridor such that the Yonge Line doesn't have to be extended into York Region.
The RH GO line is not substitutable for the Yonge Line in York Region.
Rather, the RH GO line is most useful for people in York Region going to Willowdale or Scarborough, and -- more importantly -- for people in Willowdale and Scarborough going south.
In any case, the purpose of an express line is not to bolster arguments. It is to move people.
It's only when a Yonge North LRT line starts costing more than $5-6 billion that a subway is justified.
A Yonge North LRT line, which adds transfers and reduces route flexibility without enhancing anyone's trip time, would be a waste of money.
You do however inconvenience everyone in Thornhill and Richmond Hill that rely on GO Transit to get into Toronto via a reroute.
Why would we need to take a GO train into Toronto? We can either walk into Toronto or hop on a short bus ride, depending on location in Thornhill or Richmond Hill.
As to places that the RH GO train actually goes, there is almost noone commuting by GO train to Old Cummer or to Leslie -- and using the RH GO train to get to Union is a long, slow, circuitous route. The subway makes much more sense for most.
It's only greed and ignorance that's driving the warrancy for subways to RHC, when the Yonge Line as is is leaving behind passengers on the platforms the way trains are filled by Midtown. The rationale for the subway is just not there when the multi-faceted solution of BRT/LRT/Commuter rail and subway expansion to points other than Yonge/Finch would accomodate everyone with still capacity to spare.
Part of your second statement is correct, but it doesn't follow your (nasty, and incorrect) first statement. The Yonge line is overcrowded. That's because Toronto's transit system doesn't go to enough places. By way of example, those way out in the east shouldn't need to travel all the way west, to Yonge, just to get south. However, they do. The bus routes that shuttle between Finch station and Scarborough are some of the busiest bus routes in the system. Separately, there's also enormous demand on and around the Yonge corridor itself, which is why the RHC expansion is necessary. The key to alleviating the Yonge line's overcrowdedness is to supplement it with solutions that make sense in locations where they are needed.