Relief - The primary purpose of the Downtown Relief Line to avoid a rebuild at Yonge-Bloor. The DRL is meant to relieve Yonge-Bloor station by diverting Bloor-Danforth riders from the east away from Yonge-Bloor.
I'm not really arguing with you, but if Relief was the key factor, we'd just be rebuilding Bloor-Yonge. It'd cost about $1 billion less than building the DRL, and could conceivably achieve the same results.
But they're not, they're pushing for the DRL. Why? Because it gives us an opportunity to relieve B-Y
and provide local-ish downtown service. But the two should be balanced, and I think the front/railway alignment balances that perfectly. It gives people a choice when going downtown; transfer at B-Y or take the DRL, and if the DRL was to give a clear advantage to those going to the CBD, it'd take a lot of strain off the system. I've calculated out, and a Front/Railway alignment would (timewise) be able to serve people everywhere south of Queen with a time advantage over B-Y, and I think that's pretty reasonable. A huge amount of people could be drawn from Bloor-Yonge with that, more than enough to relieve it for decades. Of course, total relief of Yonge would require the DRL to go further north than Bloor (I think Finch would be the best terminus for that,) as well as many, many, many Go improvements. But it'd easily relieve Yonge-Bloor at least.
And a Front/Railway alignment would actually get a huge amount of local (downtown-downtown) traffic. It'd be able to provide intermediate RT service between the streetcar lines in both the East and West, and would intersect a number of key areas. CityPlace, as a huge concentration of people that (I think) could be as high as 50 thousand within essentially two stations. It'd also directly hit Liberty Village and West Donlands, which are going to be two pretty big neighborhoods in a few years.
Apart from that, there would be two obvious stations in the Distillery District and St. Lawrence neighborhoods, which I'm sure would attract a huge amount of people, especially if it were to use Union Station as a node.
Oh, and it'd hit the CN Tower and Skydome, and would open up that area for people from all over the city to commute in by Subway. It'd also open up an easy route to the Ex from the West End, which would be a huge help all year around, but especially during the Ex.
A Queen alignment might hit the downtown core a lot more broadly, but there are umm 3 problems that I have with Queen.
The first is that it really just doesn't hit as many big neighborhoods as the Front line does. The Front alignment will connect with big neighborhoods like Distillery District, St. Lawrence, straight through the centre of the downtown core, CityPlace, and Liberty Village. Queen can't top that in "really big areas."
The second is that going along Queen would compromise the purpose of the DRL, smashing the beautiful glass swan that the DRL is with a sledgehammer, then putting the remains into a blast furnace. Essentially, it'd choose 2 paths: DRL or Queen line. Choosing the DRL would definitely not serve Queen the best, as it'd result in a patchy subway coverage, that quite honestly would be pretty abysmal, especially for a route like Queen.
And that brings me to the second path of a real Queen line, which'd result in very frequent stop spacing, probably similar to that of the YUS's 400-500m spacing compared to the DRL's ~1 km ish spacing. This would absolutely massacre the point of the DRL because, really, who would want to transfer to another subway line that'd just be slower than taking the YUS to B-Y?
As I've said before, Queen would really be best served by a St. Clair-style LRT making local (400 m) stops, and probably dipping underground for a short portion through downtown.
The third is that... I forgot what it was going to be. But really, marrying a Queen line and DRL is a bad idea. You might think Queen Subway+DRL, what's not to like? But really the two are like oil and water; they just don't mix. Unlike oil and water, the two could be very helpful in supporting eachother, but nobody should get the impression that two for the price of one is going to be astoundingly better. In fact, that's not even true because the rail corridor DRL would actually cost much less than a Queen line.