You could get a number of responses on this one. Big sweeping histories with big sweeping conclusions have not tended to hold up well.

They got out of date because of new research. Edward Gibbon's "decline" of the Roman Empire , for example, was written without much knowledge of the Byzantine Empire, and before archaeology revealed a more complex and long-standing relationship between Rome and neighboring barbarian tribes. Toynbee took so long to write his gigantic Study of History that some of the early volumes were already out of date by the time he was writing the last ones.

They tried to draw very wide conclusions about a lot of disparate things. When Crane Brinton wrote his classic Anatomy of a Revolution, he tried to group together the English Civil War, the American Revolution, the French Revolution, and the Russian Revolution. But, there's so much they didn't have in common that it was really hard to do. The English Civil War was not all internal revolt, also involved Scotland and Ireland. The American Revolution saw remarkably little social upheaval: the colonial elite stayed in power.

Those broad conclusions tended to follow their biases and get written to a purpose. Gibbon didn't like the Christians and thought ( without evidence) they were less tolerant than the pagans. Spengler, in a very 19th c. system-building pseudo-science way, thought of societies and cultures in terms of racial groups. And Toynbee , Spengler and Gibbon wanted very much to push the idea of a decline, a decay, a collapse, being a possible , periodic danger to what they thought of as a civilization (especially the civilization they, as elite white men, thought they were in charge of).

But often even less grandiose theories can become very hot topics, create a lot of discussion, and then just get dropped. I had a professor who had done work in grad school on the English Civil War. At that time, the hot theory was whether the landed gentry were more often Puritans, becoming more wealthy and demanding more power, pushing to overthrow the established order: the "Rising Gentry" theory, After much Oxbridge fierce feuding, the consensus eventually was that people- noble , gentry or poor, were sometimes Puritan and sometimes C of E, and people- noble, gentry, or poor, were sometimes behind the King, sometimes behind Parliament, or trying to stay hid. In other words: nothing there.

However, chop away at the grandiosity, and sometimes a bit of a theory is useful. Crane Brinton, for example, suggested that a revolution, with big social upheaval , will end with a military figure in charge: he's often been right. And theories are, obsolete or not, great good fun to talk about. I mean, if you've spent a few days digging through the ledgers of a merchant in the 1790's, until your eyes are getting dim, it's a relief to throw around somebody's big ideas. Though, I have to say, I never want to have to read Spengler again.