Contra tractability specifically, this depends on a lot on what societies count as ‘socialist’ or not. A lot of European countries, for example, are essentially market socialist or hybrid economies (large social states as in the Nordics, strong or mandatory labour unions as in Germany, large cooperative companies as in Spain, widespread state ownership of industry as in the Nordics or Poland). These countries have the longest histories of socialist movements, and have all achieved these economies through reform.
Contra tractability specifically, this depends on a lot on what societies count as ‘socialist’ or not. A lot of European countries, for example, are essentially market socialist or hybrid economies (large social states as in the Nordics, strong or mandatory labour unions as in Germany, large cooperative companies as in Spain, widespread state ownership of industry as in the Nordics or Poland). These countries have the longest histories of socialist movements, and have all achieved these economies through reform.