As with people, so it is with places, when tipping points are reached. Historical analogies for Calcutta’s relative and rapid decline rhyme and chime with contemporary Hong Kong; geopolitical tilts away from the West – in particular India’s warming ties with the Soviet Union throughout the 1960s – accelerated the pace of change.

Inward-turning, politically motivated socio-economic policies, however broadly beneficial at national level, could be profoundly destructive when imposed on cities whose ultimate value for their own country was as an “international, cosmopolitan” window on the world, geographically anchored “in” their own motherland, yet not entirely “of” it.

Chip that aspect away – or create circumstances whereby those who made a city distinctive depart of their own accord – and permanent change becomes inevitable.

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Geoffrey Moorhouse, in Calcutta: The City Revealed (1971), recognised minor but unmistakable telltale signs of this steady exodus during a late-60s research visit, when he saw for sale “bundles of electroplated cutlery, which have been abandoned by the latest British family to leave the land”.

In contemporary Hong Kong, sizeable mansions can be furnished from the leavings of today’s departing residents, often for free, simply for taking stuff away. Throughout that period in Calcutta, commercial migrants from other parts of India (mostly Marwaris) bought out going concerns whose only reasons for existence were in the city. British business names still dominate these corporate facades, two generations after their former owners left Bengal forever.

Once-global aviation hubs rapidly decline into regional backwaters when major international airlines reduce flights to a symbolic long-haul presence. When that happened, Dum Dum Airport, an obligatory London-Hong Kong transit stop, swiftly became – like Beirut and Bahrain – a footnote in aviation history.

By the 1980s, lingering relics of Calcutta’s cosmopolitanism – “seedy but proud”, as Jan Morris described them in Farewell The Trumpets (1978) – became sought-out curiosities for visiting journalists.

Outward forms of the old lifestyle remained intact, such as club bars where absolutely nothing – from dress codes and drinks ordered to modes of speech – had altered, except the ethnic origin of the men who propped them up night after night. Those few non-Indians who persisted – Armenians, Sephardic Jews, Anglo-Indians, Chinese and diehard British and other European “stayers-on” – usually remained for want of realistic personal alternatives elsewhere.

Regularly interviewed for background colour, their lingering presence further underscored how many others had left.

One long-resident Scottish friend regularly proclaims to me, as a baleful Celtic hex against doomsayers, that “no one ever made money betting against Hong Kong”; similar was said of Calcutta. Confidence was partially justified; “The City of Dreadful Night”, as Rudyard Kipling grimly labelled it, remains a wealthy place, albeit with much appalling poverty.

But the changes were profound when a critical mass of inhabitants – rightly or wrongly – felt themselves no longer wanted or welcomed, reorganised their lives accordingly, and permanently departed within a remarkably short space of time. Much of what made the place special got up and went with them, and has never returned.

When once-dynamic cities wilfully disappear up their own self-regard, the world moves on without them. Hong Kong offers eerie echoes of Calcutta’s dramatically swift transformation, barely half a century ago.