What do these rich people who gain influence at HSV, Schalke, etc. really gain from this if the club fails and becomes less valuable? Why intentionally make big promises that you won't keep? Have they bet against their own clubs? They're poorly managed but I don't see what part of that is caused by having a big fan base.
For this type of person it's all about ego. You get a lot more media hype and adoration by being the 'big boss' at Schalke or HSV than you would running a meat processing plant or a construction company or whatever else it might be. It's not like they intentionally want to do poorly, it's that they need to be little dictators and don't know much about the football business, so they can't hire professionals to do that 'solid management' we talked about.
A big fan base has a lot of influence at a club, HSV fans and Schalke fans can - together with the local media which can drive fans in certain directions but at the same time is heavily influenced by the mood in town - influence decision-making. Fans know even less about the business side than the impresarios in charge, but are often eager to get a speedy turnaround, to have someone with power and money in charge, and they're quick to find scapegoats. They breathe down the necks of everyone, upping the pressure on management and players.
Teams like Leipzig or Hoffenheim can quietly run the club like you'd run any medium sized business. They don't face burning scarves, rocks thrown at the team bus, pitch invasions, angry fans outside the club offices demanding to speak to someone in charge, or critical, alarmist headlines in the local papers, weekly opinion pieces from former players or coaches, constant speculation about the club's next moves.
One of the favorite words of beleaguered managers, directors and players at such clubs is "Unruhe" (turmoil, unrest, agitation) to describe what they don't want, but suspect third parties are trying to incite at the club. Big fan bases can cause a lot of that.