I'm also no expert in ancient history, but as far as I know it is not taught in universities that the republic was an utter failure that needed to be replaced by an authoritarian empire.
The mere fact that Rome was a republic for such a long time shows that it wasn't a failure at all.
There were several reasons for the downfall of the republic, like social issues or corruption, but there is probably none that can be singled out as the main factor.
One important point might be the expansion of Rome and its increasing influence and interventions throughout the Mediterranean. I've read historians pointing out for example how over time military leaders were given more and more power, at least in the name of their imperium (imperium in the sense of mandate or mission). I don't remember the exact technical terms, but it was suggested that the imperium M. Antonius Creticus received against the pirates and later the powers Pompeius received through the Lex Gabinia were deceisive steps that led to the shift of power from the senate to single figures.