For loops do not get the job done easily enough. I've lost count of the number of times I've had to do contortions in order to count backwards inclusive down to zero with an unsigned int. With a proper iterator API, it's trivial.
Furthermore, for loops are a pain to optimize. They encourage use of indices everywhere, which results in heroic efforts needed to eliminate bounds checks, effort that is largely unnecessary with higher level iterators. Detecting the loop trip count is a pain, because the loop test is reevaluated over and over, and the syntax encourages complicated loop tests (for example, fetching the length of a vector over and over instead of caching it). For loop trip count detection is one of the major reasons why signed overflow is undefined in C, and it's a completely self-inflicted wound.
I'm generally of the opinion nowadays that adding a C-style for loop to a language is a design mistake.
There was a nice paper at this years POPL which (in my opinion) allows you to substantiate this claim.
The paper is "Stream Fusion to Completeness", by Oleg Kiselyov, Aggelos Biboudis, Nick Palladinos, and Yannis Smaragdakis.
The actual question in the paper is how to compile away stream operations to imperative programs using for/while loops and temporary variables.
On the other hand, if we look at the output of the compiler we can see how complex it is to express natural stream programs using only for/while loops.
For instance, even very simple combinations of map/fold/filter create while loops and make it difficult to detect the loop trip count afterwards (in fact, you need an analysis just to detect that the loop terminates).
If you combine several such streams with zip, the resulting code makes a nice entry for obfuscated code contests.
Finally, if you use flatMap the resulting control flow doesn't fit into a for/while loop at all...
So for/while loops are not simpler than stream processing primitives and unless you explicitly introduce temporaries (i.e., inline the definition of map/fold/etc.) you quickly end up with very complicated control flow.
If you want to show something else is easier to use, you'd have to do a user study, and even that's not going to be universally applicable since it depends on the previous experiences of the user population being tested. It's why these things tends to be debated as a matter of taste.
They all show that the natural expression is through declarative programming and that for loop are never what comes naturally.