Social Democracy is not Market Socialism. Social Democracy is Capitalism with expansive social safety nets, not Worker Ownership of the Means of Production. This is fundamental. Again, Social Democracy is not a transitional state towards Socialism nor Socialism itself, but welfare Capitalism.
This is a description of current working social democracies which have not fully adopted socialism and still have potential in that regard. Social democracy is a collection of ideas and policies. Workers owning companies as opposed to share holders is not incompatible with social democracy even if it's not something that's being done currently at scale. Again market economies are not inherently capitalist. Welfare is also a part of the picture, but it's not enough it's own. Welfare is covering for systemic issues that have to fundamentally fixed.
Leftist theory does in fact have practical applications. Being confidently incorrect as you have been is impractical, hence the importance of theory.
Yes, it does have practical applications. I have nothing to do with the veracity of the ideas I espouse. Theory can only take us so far. I'm not interested in being limited by our current iteration of social democracy or our current definitions of socialism or social democracy. If we go with the description of social democracy in your argument, then that will be insufficient to fix our current societal problems.