Me, I think the Reichswehr had a much better reputation in general, and for stability than the unruly SA could manage.
The only problem would seem to be a shortage of manpower, and if Hitler was dead as early as the Reichstag fire, then the Arrmy could probably have bought the Steel Helmets on board.
Although without Hitler, it would seem much more likely that the Von Papen Coup would have succeeded, until someone brighter than Von Papen could take over.
Hmn... any chance of the German Social Democratic Party filling in? Otto Wels becoming Chancellor?