It is a tricky question. On one hand turning against the Legion was probably inevitable. The question lies if the Legion ever gave enough power to the Old Horde for them to ever pose a threat. Look at the lore with the Mogu and Saurok. Saurok turned on Mogu...however the Mogu were not defeated...yes they suffered casualties but the power Saurok were weaker by default of being created by the Mogu. Likewise with Burning Legion...enough power to do their dirty work but not too much for them to pose a threat.
Also we have not seen the true power of the legion. Even Kiljaden was re-sealed how Rag 1.0 was...We will face him and the true power of the legion soon enough!