In practice, all countries have a mixed economy.Hey dude, once someone actually comes up with a new system that works, I'm all ears.
It is easy to imagine a world where slightly more care and attention is paid to the most vulnerable in society. The argument against is usually that it slows economic growth. Care for the vulnerable generally means tax and regulations, but global capitalists tend to prefer low tax, low regulation systems so they can be less inclined to invest in such countries. Thus countries begin a "race to the bottom" to make themselves attractive. Even where one country may be inclined to have high social protections, the perceived threat of being outcompeted and becoming poorer relative to their neighbours is a problem. How true this really is, I wouldn't like to say. In the long run (like 20-30 years), to compare two countries of roughly similar size and position but high tax or low tax (e.g. France v Germany, Denmark v. Netherlands), I'm not sure you'd see much difference.
The USA however is mostly growing faster than the EU. This is often touted as superiority of the low tax system - but the USA is in a very different position compared to the EU in many ways, and economic growth depends on a vast number of other factors than tax and regulations. The EU is fragmented politically and economically in a way the USA isn't; the USA may benefit from considerably easier access to current fast-growing markets (the far east) than the EU, and so on.
What I certainly think is that this low tax, low regulation system is highly beneficial to the rich, to ensure that they benefit most from economic growth. Whether it's true, or to what extent it is true that low-tax low-regulation systems are better for raw GDP growth, I think this is very much a secondary concern in the political debate, and sides are actually more motivated by distribution across society.