1. If you've been following Delta's Qs, Ks, and earnings calls since the merger with NW, then you would understand that Delta's margins at DTW, MSP, ATL, and SLC (its Core Hubs) are significantly greater than Delta's system-wide margins. These hubs generate huge profits with incredible yields. MSP and DTW have almost the exact same number of O&D passengers (different mix: more int'l at DTW and more domestic at MSP), and Delta captures the vast majority of that highly profitable, high margin O&D traffic. If Delta were to close MSP and move those flights to DTW, Delta would no longer be the preferred carrier for the MSP O&D traffic and the MSP corporate contracts. Delta would face vigorous competition from UAL and AA; is there really a big difference between connecting over DEN or ORD or DFW vs. DTW? Why on earth would Delta give away all of that high margin, high profit traffic just to, ostensibly, create a better connecting complex at DTW? Delta's network doesn't need this consolidation, and it would be a massive hit to revenue.
That is not the only issue. If you talk to someone in scheduling and operations at a US airline, they will likely tell you that crews who have to commute to their bases are a significant cause of flight cancellations. Delta has a large number of pilots and flight attendants who live near MSP. If Delta closes that base, Delta now has to get those crews to DTW or another base for every schedule. A huge challenge right now, by no means the only one for airlines, is crews who commute to their bases not getting there in time to staff their schedules. Why would an airline close a hub in a city in which a significant number of its flight crew live simply to force them to commute to another base in a time of flight delays and cancellations? This would only make the scheduling problems worse, not better.
Today's non-crew labor market also makes a combination of MSP and DTW impossible to staff. Delta would need only a hand full of mechanics, gate agents, ticketing agents, and ramp staff, etc. in MSP if it moved the traffic to DTW, but would need to increase significantly those positions at DTW. Where do you expect Delta to get those new, trained employees at reasonable rates in Detroit over night? Mechanics might move to keep sonority, but Delta would likely incur significant costs. Much of the other ground staff would likely not move, even if Delta paid for it, which would require Delta to find new replacements in the DTW area. With today's tight labor market, likely to continue for awhile, it would likely take significant time and significant sums to staff a much larger DTW operation.
2. With respect to focus cities, if you follow Delta's investor presentations and other publicly available information, then you would understand what a focus city is for Delta and the two ways Delta grows and develops them. At its core, a focus city at Delta is a city on which Delta focuses corporate sales staff to acquire corporate sales contracts to capture the profitable corporate travel market.
The first type of focus city at Delta involves network growth first with corporate marketing second. LGA, SEA, and LAX are all examples of this focused growth method. At those airports, Delta decided to move into the markets aggressively with large operations first followed by the all important marketing for corporate contracts. Delta took a financial hit in the beginning using this method, but it knew it had to have meaningful networks in those large travel markets before it could steal contracts from competitors. But once it had fully developed networks in those cities, Delta was able to capture more and more corporate contracts and to convert those airports into hubs which led to an increase in market share for non-corporate travel.
The second method is the exact reverse: Delta first hires local sales staff to meet with potential corporate contract partners to determine the likelihood of securing their business from competitors and to determine what route network would be required to serve those corporate clients. Delta then adds the routes and schedules necessary to serve the corporate contracts, i.e. flying RDU to EWR for the pharmaceutical industry or BOS to DCA for the legal industry. Once those new routes are in place, Delta's market share grows to the point that it can add routes not required by corporate partners but helpful in making the operation more profitable and more desirable to the traveling public at large. In the case of BOS, this growth, anchored by corporate contracts, enabled Delta to create a hub in the city. For RDU, this has resulted in a significant non-hub network.
At AUS, it's fairly clear that the corporate contracts Delta has secured do not require any additional routes except to Europe. Post-Covid, that may change. But one method of focus city growth at Delta is to grow only with needs corporate partners at first, and this appears to be the method for AUS.
Frankly, I think the role of corporate contracts in route selection and network development at the Big 3 is highly underrated and highly discounted on this site. And certainly, Delta is free to use the term focus city however it chooses.