If I would fly enough to have top-tier status on two airlines, I would diversify as well.

Everyoneīs travel patterns are different, but for me it makes more sense to have top-tier status on one airline (one which I really enjoy flying) instead of mid-tier status on two.

Same as for Eastbay1K, domestic upgrades are not the reason why I like to have status on Delta, these upgrades may or may not happen. For me the GUCīs are very valuable - I already burned through my four GUCīs for 2024 - as well as international lounge access (writing this from the CDG Air France lounge in terminal 2F) and IRROPīs handling, even with partner airlines.

One time I needed to go from NCE to SXB for a conference, where I was giving a speech early in the next morning. 10 minutes before boarding, the flight was canceled due to a mechanical issue. It was the only flight on that day. Called the Air France hotline, and they told me to board a flight to Lyon, to connect there to a flight to Strasbourg. When I arrived in Lyon, it turned out that the flight to Strasbourg was canceled as well. So Air France got a taxi, to drive me and one other passenger over 300 miles from Lyon to Strasbourg. The taxi bill was over 1100 Euros, and without paying one cent above the 150 Dollars I paid for the original flight ticket, I got to my conference there on time. These things are invaluable and while I'm sure it can be different for others, it is the reason why at least for me it is not foolish to get status on Delta.