If you were (for example) Jet2, would you want to start services at STN in 2021 using slots leased from Easyjet, knowing that in 2023 when demand for air travel resumes, Easyjet would then just take the slots back, and fly whichever route you had spent 2 years trying to develop while (likely) making a loss ? Alternatively, Jet2 could call Easyjet's bluff, and force Easyjet to use the slot, or let them just go back into the pool - at which point Jet2 then apply for the slots which they can then keep indefinitely