In Kate and Leopold, because the inventor of the elevator is brought to modern times using time travel, all elevators start malfunctioning and eventually disappear. One of the characters explicitly brings up that if he is not returned quickly, people will vanish that would have been his descendants. Presumably the same thing would happen to them as did happen to the elevators-first they would all start gaining more severe medical problems until eventually they all died and vanished.
Also, how well is Kate going to get along stuck in the 1800s? Her job is completely impossible there, so she has no way to make a living, which leaves her dependent on Leopold (in a time without much in the way of women's rights), and if their relationship should go bad...
Yeah, Leopold should've made that clear to her. Women of the 21st century have it way better than women of the 19th century.
Well, she could've been a school teacher. I know, not as glamerous and awesome as the job she had in our time (not to mention the...oh yeah...rights she had in our time), but she would at least have a job (though it wouldn't pay her well, if at all.)
She's going to be a duchess in the nineteenth century. Why would she work?
Because Leopold was marrying for money. Since Kate left the party in just an evening gown and she wasn't even carrying a clutch purse, the happy couple are in as much dire financial straits as Leopold was at the beginning.
The media will spin the whole thing to make it sound like Kate was psycho and her company actually attempted to hire her. Because of this, they'll suffer a severe backlash economically. And seeing as the economy tanked in 2009...
Nurse Gretchen is going to either have her medical license stripped from her or jailed for willingly letting a believed-to-be-insane patient run back out into polite society, therefore endangering both himself and others.
Charlie is going to have a hell of a time trying to get a job now that people know he was related to a girl who seemingly killed herself and friends with two men who seemingly drove her to kill herself.
How plausible is it that nobody else would've dreamed up the elevator in Leopold's absence? (In the real world someone else did invent it. Modern-style safety elevators were actually fairly common in New York City by 1876.)
If Stuart is Leopold's grandson and Kate and Leopold wind up marrying in the 19th Century, doesn't that mean that Stuart spent two years boning his own grandmother?
Being from the Victorian era, shouldn't Leopold be much more sexist? And at the end when Kate goes back in time to 1876, doesn't she basically give up all human rights, being a woman and all?
Women had rights in the Victorian era, though not many. Women couldn't even vote yet despite the fact that the monarch was a queen.
And also, not every man in the 1800s treated women like shit.
Kate will also at least have the benefit of being a duchess by marriage, a position that, while still very limited in comparison to the modern day, offers her much more freedom than the average woman of the time would have. She's still kind of screwed if the relationship with Leopold doesn't work out, but since it's a romance we're obviously meant to believe that it will.