Happiness as a function of money/power/etc is a logarithmic function. Given that there is a finite amount of these things to distribute among a given number of people, the distribution that will maximize total happiness of the population is one where everyone is equal. That isn't theory, that's fact, with mountains of evidence.

Marxism is the strawman that the right wheels out to try and push back against policies that encourage equality. Equality doesn't equal marxism, proving marxism won't work doesn't prove that policies for equality won't work.

One point I concede to conservatives is that being too strict about equality dampens personal autonomy (which is very important to happiness) and reduces incentives to innovate. A balance must be found. I should note that happiness from autonomy follows an inverted U curve, where too much autonomy (e.g. too many choices) causes just as much unhappiness as too little. Additionally, many extremely wealthy people don't spend in proportion to their earnings, the money is just their score in the game of capitalism, so redistributing more of their earnings doesn't materially affect them as long as you are doing the same thing to their competitors. Based on these facts, I believe that the autonomy/equality balance is currently very sub-optimal in terms of happiness.