Not really because who is going to fly YYC-ATL-BOS/NYC/BDL/PVD/DCA and the like these are all accessible over Detroit better than ATL. Just because it's a bigger hub doesn't mean it's more convenient..
But there will be enough people flying YYC-ATL- JAX/MLB/EYW/CAE/GSP/MYR/SAV/BHM/JAN/HSV/BQK/GNV/TLH/GSP/any other SE US destination or onwards to countries like South Africa, Argentina, Brazil, Colombia, Peru or Caribbean / Mexico / Central America destinations that WS doesn't serve to make it worth while. There isn't a lot of traffic between YYC and ATL, but when you factor in the power of the DL ATL hub, where you can basically make any city pair work, YYC-ATL will be done by WS and will be successful.
For connections between YYC and the NE USA, passengers will be routed through YYZ and then onwards.
Please all those places you mentioned on a good day probably generate 20 O/D a day. WS doesn't serve BDL,PVD,PHL,PIT,SYR,ALB,EWR and the like so hence DTW is a much better add.
You would be surprised. WS is doing YYC-BNA 2-3 x / week, and occasionally throwing -800s on the route due to high loads. If they can fill a 737 twice per week YYC-BNA, I'm confident they have the drawing power in Calgary to get enough passengers to justify a daily YYC-ATL. And yes, I do realize that BNA is the kind of market that draws people, kind of like a LAS, whereas ATL is all about connections. I just believe in the synergies of the JV and that they will pull enough passengers with connections to fill a 737 on the route.
Look at how the AC/UA partnership has evolved, where AC has multiple daily flights to all the UA hubs from their Canadian hubs, even ones that on the surface don't necessarily make sense. It is all about the connections they enable. I don't think the question is "Will the WS serve YYC ATL or YYC DTW" - I think the JV will enable both, on the right metal. And mark my words - these carriers have enough airplanes to get the right one for the job on it.. Will be interesting what they deem the best fit, be it a WS 737, a Delta narrowbody (717, 737, Airbus 320 series), something from a Delta Connection carrier, or an Encore Q400. Offhand - how do CASMs compare between WS and DL. I would think YYC ATL would be a 737 a day to start, but YYC DTW could easily be a Delta Connection E175 multiple times per day for enhanced NE connections. The passengers exist, as AC's US route network demonstrates. This will just give them another, possibly more direct, and maybe cheaper way to get to where they need to go. And were I from Western Canada, I'd rather connect through MSP or DTW, do pre-clearance in YYC vs YYZ, plus not be stuck on an Air Canada Express B1900s or DHC-8s that seem to do a lot of the NE destinations.
Interestingly, of the flights between the WS and DL hubs / focus cities, I can't think of any current overlap between the two.
WS does YYZ LGA, YYZ/YUL/YHZ BOS (WS Encore), YYZ LAX, YYC JFK and YYC/YEG/YVR LAX, along with seasonal service between YWG / YQR and LAX.
DL does ATL YYZ, DTW YYZ, MSP YYZ, MSP YYC, SLC YYC, SLC YVR, SEA YYC, SEA YVR, ATL YUL, I think a season JFK & ATL to YVR plus service to several secondary Canadian cities.
Also - as Jimbo737 likes to occasionally mention - DL is quite aware of how AC's US expansion is poaching passengers for international connections, mainly thru YYZ but occasionally through YUL, YYC and YVR. With WS 787s showing up in the next year or so, will be curious if the JV starts to route low-yielding traffic on WS to Europe, to compete with AC Rouge and other lower cost carriers. WS already has been pulling US connections for some of their European flights (eg FLL YYZ DUB) - imagine adding Delta's pull to the equation.
Looking forward to watching this unfold.