That doesn't follow. You can have a well-designed system where the PHP code only contributes 10% latency and resource usage, in which case the PHP code is clearly not the most important factor.
"This might just indicate you better have 25 app servers vs 100 databases and spend the rest of money proportionally on both clusters."
Exactly. What matters is proportion of latency or resource usage contributed by the PHP code, and that can vary a lot between well-designed systems. The property of being "balanced" doesn't automatically make your PHP application cluster "the most important factor".
And regardless of language difficulties on your side (which we should try to overlook), the conclusion that app server performance is more important than database server performance is left completely unfounded.
"if you need to use 100 mysql servers, you better darned use 100 computing servers of equal computing power, because if you only use 50 you're not balanced."
The second does not follow from the first.
"A well designed system is balanced: when the input load increases, all parts of the system take the hit."
"Given this reality, the processing power of the application cluster is arguably the most important factor."
It's just nonsense anyway your look at it, really.
Oh my. Where do we begin? I guess, using this logic, the language that is just the exact amount of slow to match whatever database workload you have is the optimal language.
Because, as we all know, all parts must be equally balanced, otherwise it's not optimized. So if you need to use 100 mysql servers, you better darned use 100 computing servers of equal computing power, because if you only use 50 you're not balanced.