The value of the land occupied by SOU is, I agree, likely more valuable than SOU's future profits. However, the change of use of the land would mean a decrease in the value of land all over Southampton and south Hampshire and damage to the associated economy. This decrease in value would perhaps be larger than what AGS are missing out on from a major land sale
It is in the interest of local Govt to refuse permission for a change of use while the airport seems to be self-sustaining or at least able to survive on its reserves