A faster run-time to Union Station. If you take the Yonge subway in rush-hour, almost everyone who was on at Yonge, has gotten off before Union. And of those that remain, many continue around the loop to St. Andrew and Osgoode.
If you move the Kennedy->downtown demand to GO, most people coming off the GO from Agincourt will be trying to head north somehow. And it can easily be 5 minutes from the time the train stops until you walk into the subway station, let alone get on a subway train.
Well, we have ~$3B to spend on making the best trip downtown (equal funding to subway option). I've allocated $1B for TTC to subsidize close to 1 million GO train trips over the coming 3 to 4 decades (roughly 10 minute frequencies, assumes $0 revenue for those trips and the $1B earns nominal interest).
I put $500M into Scarborough stations to create bus terminals at Agincourt, lets include Milliken too, and make substantial improvements to Kennedy integration. Perhaps we slice $100M out to put in BRT on a few congested areas to make the feeder buses flow faster since very few residents live at any of the proposed LRT, Subway, or GO stations.
Can we take the $1.5B remaining and improve the transfer at Union Station? That would buy electrification of the line, a few km of tunnel through downtown, and a nice interchange station. Perhaps we could put an interchange at King Station instead of going to Union, though that's not helpful for South Core as a destination (duck underground near Cherry St and run across to Yonge/Bay near Colborne/Melinda or Wellington).
My entire point is that arguing about trips to downtown for the subway option is dubious. $3B can create a far nicer downtown trip for more people in the same starting area. There are perfectly good reasons to build LRT or subway in Scarborough; a primary use of trips to downtown isn't one of them.
I didn't even need to get into the possible side-effects like less load on Yonge line in peak direction/peak period and the possibility York Region might kick in additional funding for that kind of downtown express plan to further improve it.