The Cubs don't have to win. In fact, it's better for the management/ownership if they don't win.
They sell out all season long whether they win or lose (contrast that to their neighbors on the South Side). They have among the best nationwide television coverage of any team (thanks to the WGN superstation). So winning won't increase revenue significantly. They sell tons of licensed merchandise.
However, if they win, expenses will go up; when a team wins the series, the players all want raises, and many have bonus clauses in their contracts; so winning a series can cost a team tens of millions of dollars.
They can maximize revenue by making it to the first round of playoffs once every three to four years. Any better than that, and expenses increase but revenue doesn't.
BTW, introducing night games was stupid. With their coverage on WGN, and a season of 3:30 p.m. games, every kid in Montana or Kentucky could come home from school, flick on the tube, and watch the Cubs. Those kids will grow up to be Cub fans. Day games assured that the Cubs would have the largest fan base in the country; going to night games punched that meal ticket full of holes.