When did the Roman Empire fall (your thoughts)?

[Theodosius the Great]

Or because Caesar was a better general.

*sigh* I hope that wasn't hero-worship. Caesar faced reversals fighting Pompey as well. Sure, his generalship was superior but not the only factor. Otherwise your "numbers" argument makes no sense.

At Pharsalus, Pompey had all the advantages in position, infantry and cavalry. But Pompey, tellingly, relied on his cavalry to win the battle. After his cavalry were routed he still had superior infantry but he knew he couldn't count on them because they were inferior fighting material.

I don't think civil wars necessarily degraded combat power, provided there were ample replacements.

Other factors to be considered are duration, frequency and scale of civil wars.

~Theo