The AS deal is a frequent flyer deal, you don't invest $450m in another carrier because someone else signed a FF deal with a small competitor. Think bigger. Shanghai is THE largest business market in China, and MU together with FM pretty much control Shanghai.

MU joined Skyteam because of "guan xi" (its executives were close to CZ's executives - CZ was in Skyteam before MU - so they joined Skyteam and ditched oneworld. MU had a relationship with AA before but once it went to Skyteam, AA and oneworld pretty much lost direct China access through the China Big Three and now depend on CX/KA for most of that). That goes to show that relationships with Chinese carriers can be fickle without some investment (and I don't just mean $$) or "skin in the game". Hence, DL has been grooming the relationship (including executive exchanges, training, etc.). And it has been known that MU has been looking for a strategic partner (SQ tried in 2007 but failed). If a non-DL friendly carrier got a piece of the action, it could very well mean that DL loses significant access to the most lucrative Chinese market. CZ would be the remaining option but CZ is focused on CAN which is really of not much use; even though CZ has a hub in PEK, it is far smaller and could never replace what DL is getting out of PVG with MU.