Theory: Challenges are temporary because they are tools to collect gameplay data

It's very possible.

Or it could also be data collecting to see how many people actually freak out and do the timed challenges within the time frame so they can gauge how successful future timed events would/could be in TS5 if they are thinking about making it online and having more mobile game aspects.

Because a lot of people suffer from FOMO syndrome (Fear of Missing Out) and that's what game studios bank on when they put out those limited time only events that have "special content" offered to those that complete the event within the time limit.

But typically in those types of events, completing it is almost impossible to accomplish unless you upgrade this or that (which costs real life money to upgrade or is a seriously annoying grind fest, so people end up just spending the money to upgrade). So they are making money from the people who upgraded because they're worried about missing out on some little exclusive trinket.

That's my theory. Timed events in an offline game doesn't make any sense to me.