What you wrote is not a routine for size. Period. It's imbalanced, low volume, and low frequency. I don't believe that some pushdowns and curls were the actual cause of whatever "burnout" you experienced. There's certainly room in a productive and well-designed routine for direct arm work.
Not calling out out, but can you specify what "great results" you've seen? How long have you followed this program and how has your bodyweight changed? What's your current height, weight, and general fat level (just a description, not a %)?
If that was "logical", why not train every muscle in one day? Building muscle just doesn't work that way.
Like GMoore said, you're approaching this from a flawed perspective. No offense. Symmetry comes exactly from giving everything the attention needed to produce significant growth. The overwhelming majority of people will not see "maximum mass" in their smaller muscle groups (arms, shoulders, calves, etc) by doing only compound exercises. Some people with fortunate genetics may see good, maybe even impressive, growth by training with only compound exercises, but odds are that ain't you or me or most people here.
You're very mistaken if you think John Grimek avoided isolation exercises. Like every successful professional or amateur bodybuilder, he put hard work into both compound exercises and isolation exercises, and reaped the rewards.
It's not so much that you're "neglecting" them, it's that you're understimulating them by avoiding direct training. It's like putting lettuce and tomato on a burger and saying "well, I got all my veggies in for the day." Nope. Because it's simply not enough.