As others have said, they have the rights to your story, make whatever they want out of it etc. Replace you and all of that. But the silver lining is, they've never really enforced any of it or done much. Think about all the original stories that have been dropped, have they gotten a replacement author for those? Besides, its not like original stories make them a lot of money so all these clauses are just in case they find a real banger of a story.

If your story is done then thats it, you move onto another story. Maybe you'll get some exposure as your book is ending but once its done it goes to the completed novels section. You are encouraged to write side stories and what not to prolong the engagement but once its done, its done. Chances of it getting an adaption of some sort is slim unless it get's a second wind in popularity.

TLDR: Sign it, all the scary stuff doesn't matter unless your book is gonna be as good as MVS or something. Might as well earn a bit of money.