Quoting MSPNWA (Reply 51): I calculated the numbers ... MSP is still bigger even with the larger decline  about 52K seats to 49.3K seats per day.

Your calculations are wrong. Using FSDan's data,
DTW has approximately 1,500 more seats than
MSP, making it 3% larger. On a weekly basis, the percentage will be slightly larger as
DTW's daily service is slightly more volatile than
MSP's (including stronger service on Saturday's). On an annual basis, the percentage is comfortably larger; outside the summer season,
DTW enjoys a significant edge in seats vs.
MSP.
Quoting MSPNWA (Reply 57): The DL operations alone are what I totaled up. So MSP continues as the secondlargest DL hub, but the gap with DTW narrowed a bit.

None of this matters, anyway, since industry standard for size is ASM and RPM of which
DTW is significantly larger than
MSP.
Think of retail: when you read 'Axe deodorant has more than 80% market share among young males,' do you interpret this as fouroutoffive deodorant products sold are Axe products? Or do you recognize that when one person purchases a Speedstick product (average retail: $1$2) and the other Axe (average retail: $4$6), Axe reigns with 80% revenue (market) share? Revenue is the gold standard.
Thus,
DTW is and will remain
DL's secondlargest hub.
We don’t care what your next flight is.