Or maybe, just maybe, they think that taking the rugby income, being guaranteed, is a teeny, tiny bit more financially prudent than hoping against hope that a slightly improved pitch will turn average players into good ones and in turn bring back lapsed fans who have only given up because the standard of football just isn't good enough. It's not about what income is more important, of course if we were guaranteed to recoup our money from supporters by dropping the rugby and letting the pitch improve we would, but we're not, and it's utterly ridiculous to suggest that it's more 'important,' I honestly can't fathom what you mean with that nonsense.
Other than that, your thoughts have been ably answered by lmj.