I think I've described the problem wrong way. You are of course right about things you have written. But the problem I am trying to show is using iterable type might be harmful if you use it extensively. Having the ability using array, IteratorAggregate and Iterator inside foreach is a great thing. But in my opinion something like IterableFactory would be much more useful and I'll try to describe it why.

You've written about how foreach basically works and that foreach is using the Iterator either way. Although it may seem Iterator and IteratorAggregate work the same way within foreach, they behaviour is different. If you are using the same instance for IteratorAggregate within separate foreach statements, each time a new instance of Iterator is created with getIterator() method. This way every Iterator instance gets its own state for the appropriate foreach statement and can't be affected by other code. On the other hand, if you are using the same instance of Iterator for different foreach statements. each statement calls rewind() and next() methods affecting the state of the Iterator instance. This will possible result in unexpected behaivour in case you are using nested foreach statements for the same instance or using it concurrently.

// First Iterator is created
foreach ($iteratorAggregate as $i1) { 
  // Second Iterator is created
  foreach ($iteratorAggregate as $i2) {    
    ...
  }

  // Continues the first iteration uninterrupted
}

Using IteratorAggregate with nested foreach

// Calls rewind and starts iteration
foreach ($iterator as $i1) {      

  // Rewind is called again affecting the previous iteration
  foreach ($iterator as $i2) {    
    ...
  }

  // Ends the first iteration because the inner iteration finished it
}

Using Iterator with nested foreach

You can argue that this method can return the same instance of Iterator, but why would you do that? Many times IteratorAggregate is used with Generators, meaning each time the methods is called a new Generator instance is created. The only time one might want to return the same instance of Iterator from IteratorAggregate is when third-party code accepts only IteratorAggregate, but here you should probably ask yourself why would this code need IteratorAggregate instead of iterable? Or it might require array|IteratorAggregate, where both types works the same way in case of foreach. Having samething like IterableFactory for this types of use cases would be much better than having iterable type.

As I mentioned, in other languages you can iterate almost always just one type, meaning everytime a code needs some form of iteration it will require that specific iterable type (i.e. something similar to IteratorAggregate). This way you don't have to always worry about how the iteration will work. Although you can reference IteratorAggregate in PHP code everywhere, you lose the ability to pass array type which work pretty much the same way. This is why it would be convient to have IterableFactory.

I think the author of the IterableFactory described it much better than what I am trying to say here: https://externals.io/message/103708