I think it would really need to be one or the other. Socialism and Capitalism are really not compatible with each other. Some would likely say that they are polar opposites of each-other.
Absolutely true! Mixed economy welfare states will definitely fail on the long run. Look at the EU now and compare it's economic performance to the "more capitalist" U.S., Hong Kong, Singapore, New Zealand, Taiwan.
Capital owners will simply fly away from the countries with high welfare(=tax) burden.
Therefore at some moment in the future only minarchic capitalist and pure socialist (Marxist) states will exist. And no one between them!