There are so many studies showing that social and financial inequality are bad for everyone's health, happiness, and wellbeing, and actually bad for the economy as a whole. There's never been a study that I know of that shows the opposite.
The standard of living getting higher (a trend we've definitely bucked for a few years now by the way - life expectancy has fallen for example), does not mean people are happier. What's definitely true is that since the 70s economic productivity increases have been decoupled from wage increases, and inequality has increased massively. You'll note the biggest increases in living standards in my lifetime came under New Labour, and they were forced to throw in the odd left wing economic policy such as the minimum wage from time to time. The biggest win for people and the economy under the coalition government was bumping the tax free threshold up, also a progressive form of tax cut rather than e.g. cutting the top rate of tax or increasing VAT.
I do agree that most change is better to be gradual, but we desperately need it to be leftward right now given the state of the world, and we can't afford for it to be too slow.