the stadium hire business does not make up for drop of revenue from bad performance: I don't know how much it would be to hire the pitch for sports and concerts, but from the looks of it the PHEX event was in the South Stand for something like £10k per day.
Assuming £5m-£10m in stadium income, compared to the risk of dropping 4 places by neglecting the squad:
£9m less in PL prize money.
Tens of millions less from not being in CL.
Probably lower performance payments from the sponsors.
Lower returns from the corporate box holders.