Any business depends on cash flow for its existence, especially entities like football clubs. Many businesses that go to the wall are not necessarily insolvent, they just have inadequate cash flow. Cash flow enables a business to manage things like debt gearing - provided you have enough cash coming in to the business to enable you to service the debt, you don't have a problem in theory, and that is the zone in which the majority of football clubs exist.
Season ticket sales give clubs the cash flow that piecemeal match-by-match ticket sales can never guarantee, and that is why clubs offer a discount for them. The argument that a club would make more money by selling fewer season tickets is largely a red herring on that basis because it introduces too much subjectivity into the budgeting process. Add in the possibility of a bad winter with home games postponed, and potentially you have a problem paying your bills because you have no cash coming in. Good season ticket sales may create lower revenue if a club has a very good season, but it does enable them to manage what they have more efficiently.