Seems a bit shallow.
The real downfall of the great British houses (architectural sense) was the financial catastrophe that 1914 to 1946 was for the British Empire. Top billing in two world wars - and they went from being blatantly the richest and most powerful nation on earth, to needing a US Treasury bail-out to avoid national bankruptcy.
(Though over in America, most of the historic grand mansions are now tourist attractions, for lack of heirs with the wealth and interest in maintaining them.)
And the benefits of British aristocratic titles faded over quite a few centuries, not just recently. Compare King Charles I of the early 1600's (Parliament didn't like his exercise of Divine Right) with George III of the later 1700's (a clever King could appoint his own Prime Ministers against Parliament's wishes) with Queen Victoria of the later 1800's (she complained to the PM that the Foreign Secretary was taking actions without her approval) with Queen Elizabeth II of the later 1900's (she dutifully read her supposed "Queen's Speech" to Parliament, whether she agreed with a word of it or not).
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