Schalke 04 now are bad as Weder Bremen after they lost their key players and got failures with their transfer strategies. It is net effects: loss of key players, bad results, failures in transfers, bad results. All of those cause bad income and smaller transfer budget. After few seasons, they got loss of directions and reach the edge of demotions.
Schalkes situation is probably worse than Bremens. Schalke has way more debts than Bremen, even though Werder is in a bad financial situation, too. But even more important: Other than Schalke, I don't think Bremen will relegate this season. And when you are in such a bad situation financially like Schalke, relegation hurts even more. The best players will leave the club (Kabak, Serdar, Harit) and they won't get as much for them as they have hoped for. So they would need to build a new team from the scratch but don't have the money to do so. And the second Bundesliga is no walk in the park. Just look at the HSV. An instant comeback to the first Bundesliga isn't guaranteed. And then you have to learn to live with much lower TV money etc.
But this is all hypothetical. Schalke isn't relegated, yet. Even though I don't have much hope for them. But maybe they'll follow Werders example from last season and trust on two teams doing even worse (Bielefeld, Mainz??).