Social democracy is inherently unstable but so is liberalism.

Obvious - Social democracy as per 1945 is an inherently unstable compromise because in a capitalist economy it favours economically the politically weaker side, i.e. the workers.

Not so obvious I think - Liberalism as in the mid-nineteenth century UK is inherently unstable because in a capitalist economy money naturally flows uphill where it strengthens the political power of the capitalist elite i.e. it favours the politically weaker part of the capitalist class.  Once some part of the capitalist class finds itself able to use the power of the state to its advantage, liberalism is doomed.