The SA could have done nothing. They were a bunch of overweight bullyboys and thugs, half of whom were socialist.
Hmmm ... depends on how you define "socialist". They certainly weren't lefties. The SA emerged immediately after the Freikorps were disbanded, and were mostly composed of ex-Freikorps. Their ideology was basically the same: they believed in the Dolchstoßlegende, that Germany had lost WW1 because they had been betrayed by the November Revolution parties who were (according to Freikorps beliefs) led by Marxist Jews.
Freikorps had been formed by the old Reich to combat groups like the Spartacists, after an incident when Wehrmacht units refused to fire on a crowd of communist protestors which included women and children. The idea of the Freikorps was to form paramilitaries with radical right-wing ideologies who could be counted on to attack revolutionary elements.
After the revolution, the new Weimar Republic had no use for them and started disbanding them. This caused the Kapp Putsch, which failed, and was followed by the wholesale disbanding of all Freikorps unit. A few months later the SA was born.
This element was only socialist in the sense that they wanted government to step in and liquidate foreign and Jewish businesses and do a few other things like increase public spending for the military, clamp down on business that went against conservative values (ie cabaret, swing, and jazz clubs), and so on.
The well trained and efficient Wermacht would have destroed them in any case.
Not so sure. Using the Wehrmacht against Germans was problematic during this era, regardless of whether the targets were left or right wing and regardless of the political administration. The Kaiser, the Weimar Republic, and even Hitler all experienced problems trying to do this. That's why the SS was used to round up the SA, not the Wehrmacht.
Not that there was any love lost between the SA and the Wehrmacht. The Wehrmacht, led by Prussian aristocracy, despised the SA as a loud-mouthed, degenerate mob which threatened peace and order, and they were disgusted with the notion of having to incorporate the SA into the army (which the SA loudly demanded, regularly). The SA despised the Wehrmacht, feeling they were too liberal and not committed to Nazi ideology. Nevertheless, the Wehrmacht was not sufficiently antipathetic towards the SA to overcome their distaste for killing Germans.
Had the Night of Long Knives failed, and the Nazi leadership destroyed, the far right in Germany would have been in disarray. The KPD would probably re-emerge and seize power, and undoubtedly Ernst Thalmann would have been released, Pieck and Ulbricht would return from exile, and the German state would fall in line with Stalin.
