Inequality gives rise to populism and extremism all the way up to civil unrest. If the middle class ignores the woes of poor it is swept by revolution aimed at rich.
Inequality makes possible our current situation, where the owning class encourage and exploit immigrant labour, only to disseminate "news" aimed at making the working class hate those immigrants.
Same story as in plantation times: Make the white servants feel superior to black slaves by virtue of skin color; manipulate poor whites into believing that any perceived gains by blacks had come at their expense.
Although the absolute wealth of poor might have improved since 1900, people can still be angry that someone is extracting disproportionate amount from the system while their situation is stagnating. See flatlined real wages since 1970s
Like many pointed out here, this "43%" was nothing special this year if, for example, you are mid-career and have moderately aggressive stock market participation. It's an example of headline entirely cooked up for agitation.
You are right, it's not the majority of the population. The fine article was trying to raise indignation at the wealth increase of "the 25 richest families in the world", using seemingly gross numbers that they militarized without even noticing that these numbers were completely unremarkable. Or perhaps in bad faith altogether.
Paywall link Paywall link Makes a slight case but mostly just assumes inequality is bad.
With better outcomes for society: labor movement at the turn of century and in 30s.
More recently: Arab spring, Chile 2019
If the French revolution wasn't about income inequality then there has never been a conflict about inequality... Read the Rosseau and Voltaire of the period leading up to the crises. You can feel their passion when they talk about inequality.
Here's just one event from the time: https://en.wikipedia.org/wiki/Women%27s_March_on_Versailles
"The rioters had already availed themselves of the stores of the Hôtel de Ville, but they remained unsatisfied: they wanted not just one meal but the assurance that bread would once again be plentiful and cheap. Famine was a real and ever-present dread for the lower strata of the Third Estate, and rumors of an "aristocrats' plot" to starve the poor were rampant and readily believed.[2]"
Not about income equality per se, but you know, not wanting to starve in the streets. Lol what the fuck.
So like I say "inequality" doesn't even enter the broad collective mind. And the intellectuals that try to run "what's next" do talk about "égalité" but again that's not what they mean.
This is not what anyone means by "inequality" now. Not killing the economy with random wars, yes. Welfare, yes. Price controls even, sure. Better planning (because we are talking about famines here in this specific case - not even taxes.) Even when "Egalité" and "Fraternité" make it into foundational texts, soon after, this is not what they are about.
"Although the 18th century was a period of increasing prosperity, the benefits were distributed unevenly across regions and social groups. Those whose income derived from agriculture, rents, interest and trade in goods from France's slave colonies benefited most, while the living standards of wage labourers and farmers on rented land fell."
Same for Arab spring, it's like second sentence on the wiki. It's also heavily about corruption, but guess what, those two go hand in hand. Open maps for corruption and Gini index and you will see strong correlation.
Of course there are more reasons. Society is complex.