A few points I would like to make.
Firstly, I would argue that the ATPL syllabus is not as hard as A-Level in terms of difficulty, it certainly isn't 2nd year degree difficult. However it's the work load that kills you (significantly higher workload than a dgree). If you're not willing to put in upto 10-12 hours per day then you're going to struggle. That comment is obviously subjective and varies greatly depending on your own ability.
Secondly, I was a full time ATPL theory instructor for 3 years. In that time I saw many hundreds of students pass through my classrom. Although it sounds obvious, there was a clear correlation between those students who didn't put the effort in and those who couldn't pass exams. You name the excuse, I've heard it.
Slightly more on topic, the one thing that I noticed more than anything, were the modular students who came to our school who already had a PPL but clearly paid no attention to the theory whilst flying.
If I had a pound for every PPL student that came to me claiming they had never heard of the Lift equation, or Bernoulli, or a great circle track, or how to read a TAF, or what specific gravity is, or what TODA/TORA mean, or what Hypoxia is (the list is endless) I would be able to recoup the cost for my PPL.
I had students with A-levels in Maths that couldn't do trigonometry, and students with A-levels in Geography who had never heard of Lat and Long.
Students who relied on SkyDemon for their hours building, having taken the easy way out of using a map and a stopwatch usually struggled like crazy whilst doing their CPL.
It was clear which students read and understood the material, and those that used question banks. Those that memorised answers, or concepts but never really understood them were the ones that couldn't pass exams. And ultimately dropped out of ground school costing them thousands of pounds for nothing.
I always told my students, "It's easier to learn one concept that can answer a hundred questions, then memorise one hundred answers". You'd be surprised how many students ignored that advice because they couldn't be bothered to learn a single difficult concept.
To the OP. The background knowledge you learn for the PPL (if learnt correctly) will make your ATPLs 70% easier. The ATPL syllabus is simply an extension of the PPL syllabus, in far greater detail. The background knowledge you will have will allow you to sail through the ATPL exams whilst still having the understanding that you will require for a technical interview at a later date.
I won't deny that a large percentage of the ATPL syllabus is out of date and not relevant to the 21st century. However, the basics required for PPL will not only make the rest of your training easier, but they'll save your life too. Don't ever take the short cut, learn everything you can.