What changed about English estates (like Downton Abby) that caused them to fail after WWI?

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The changes in social and political climate that led to this first started in the 18th- and 19th-centuries. At a time when almost all politicians were landowners and all landowners politicians, a significant portion of the British political establishment was made of the Whigs, who opposed the power of the monarchy and called for a more powerful Parliament. Eventually, with changes in demographics, Whigs began calling for the expansion of voting rights, which would later on directly lead to the diminished power of the House of Lords.

However, I don’t think it’s accurate to say that early Whigs simply didn’t have the foresight to see that their decisions would ultimately diminish the power of their own (i.e., the landed gentry). Many of their decisions were influenced by the ideals of the Enlightenment period, and at least some of the landowners genuinely appeared to care about bettering the lives of their tenants:

It’s definitely a fascinating series – interesting enough for the architectural insights, but made even better by the detailed look into the people and history of the homes and estates.

I believe the episode you’re referring to is the one on Wentworth Woodhouse – this was my favorite of the entire series, and relevant to this OP. Cruickshank goes into some detail about the political decline of the Fitzwilliams family and how their political work in favor of progressive ideals of an expanded voting population led to the decline of the country estates as political machines.