Social democracy is to placate the left, not the right. Late 19th century and early 20th century western Europe was fraught with civil wars from a lot of attempts and failed communist revolutions, such as in Finland, Spain, France, and Greece. Tensions were very high and social democracy spread as a way to provide just enough concessions to working class people that the ire from the working class would not erupt into revolutionary violence.

Although, with the turn of the 21st century, with basically the complete non-existence of socialists and communists as a global force, this placation has become rather pointless and so these western European countries have begun boiling the frog and slowly rolling them back through endless austerity.

The right is never placated. It's impossible to placate them. They always want more more more. If you give them an inch they will take a mile. It really stems from the material basis of the left and right.

The right represents the ruling class, which are well-organized and backed by enormous centralized financial institutions. The left represents the working class, which are not organized at all and not backed by any centralized institutions.

This distinction matters because it means the right always, continually, behaves as a well-organized force, constantly pushing its agenda non-stop, but the left only really can push its agenda in rare instances where working class people happen to spontaneously come together enough to push back. The right is thus always at an inherent advantage in this way.

These spontaneous coming-togethers, though, tend to be short lived and disintegrate over time. Hence, if that "pushing back" doesn't lead to an overthrow of the capitalist class, and the transfer of these major centralized financial institutions out of the hands of the oligarchs and into the hands of working people, then any gains become short lived as eventually the spontaneous working class organizations will either disintegrate or become defanged and ineffective over time, allowing for whatever gains they achieved to be rolled back.