Empire is not merely about economics. When the British acquired India, they had to control the route to India (Gibraltar, Suez, Cape Town), establish and control coaling stations on the way to India (e.g. Aden) and deal with the threat of other empires like the Russian Empire.

So some colonies are acquired because of the economics; others are acquired due to security considerations. E.g. if you don't control Suez and Cape Town, you could easily be cut off from India.

Many African colonies weren't acquired because of the economics.

British colonial governors like Lord Curzon had to do geopolitics. Russia was a threat to British India - she could move southwards via Afghanistan. So British influence in Afghanistan was critical. Britain also had to extend British India to Burma to protect India from France.

Thailand was left as a buffer state between British (Burma) and French (Vietnam, Laos, Cambodia) possessions in South East Asia.

So why does France still keep an "empire" in Sub-Saharan Africa? It is not about economics & certainly not about a "sense of noblesse oblige".

To understand France's geopolitical strategy, you need to understand that France (especially under De Gaulle) never fully accepted America's "Bretton Woods" system - i.e. the US Navy guarantees the global commons in exchange for dependence on the US based system.

France was to have its independent military deterrent (unlike UK, which depends on US to a large degree). But that wasn't enough; France was also to have its "sphere of influence". After Algeria & Vietnam taught them salutary lessons, Sub-Saharan Africa was the only place left.

You'll understand what France is up to if you ask yourself this question; "if the US decides to disengage from the rest of the world, in what position will they be"?

I.e. what happens if US decides to really do "America First"?

Well, France already has its military assets in position to obtain crude oil from places like Gabon, uranium from places like Niger and iron ore from places like Mauritania.

They can get all the natural gas they need from Algeria.

France has never fully abandoned mercantilism (after all Jean-Baptiste Colbert was French). They never fully signed up to the Anglo-Saxon liberal economic model ("Dirigisme" is not going anywhere).

They never fully abandoned thinking like an empire either.

Charles De Gaulle was obsessed with not depending on the "Anglo-Saxons" (British and Americans), so he laid out a geopolitical strategy which aimed to accomplish just that - which has been largely adhered to ever since.