Happiness is good. That is my base presumption. We all have to presume something somewhere, and that is my presumption. I hope I don't have to explain to you why I might think things like health, security, leisure, abundance, and so forth would make people more happy. If you ask me to, it will be very obvious you are entirely in point scoring rhetoric mode and I'll just move on.
If we take some of these things I listed, what will basically happen in your social darwinism setup is that the distribution of prosperity will become hyperbolous. Unchecked, unregulated, those who have will get. Wealth will give one the power to obtain more wealth. Before you know it, a ridiculously disproportionate amount of wealth, and in turn prosperity in the form of those things I mentioned, is in the hands of very few. And this begets its own problems, because this further enhances a power disparity and you get some kind of kleptocratic despotism (a shitty government!).
So that's how it would be worse for most people. The majority of people would have less health, security, leisure, abundance, etc... Ultimately less power. Might be a reasonable guess that most people will be pretty unhappy at this point, too. The majority would be sub-par and a very exclusive few would be very super-par.
You getting what I've roughly said here?