Drupal uses objects. Reasons are probably lost by now in the ancient past.
PHP is still not the best. I can write:
somefunction(array('this' => 'that'))
while there is no similar construct for objects.
Also we use arrays for links, menus, links and soon mails.
So, I propose removal of objects on practical grounds: shorter notation and a very little memory gain.
Add to that consistency in the code, shorter learning curve for newbies as well.
If you have an opinion please share but I think we are only furthered by _practical_ viewpoints. No theory can make Drupal faster or lower its memory usage.
The change looks big but it's not, it's scriptable and I can whip up such a script in a very short time.
Ps. Readability issues? I do not think there is any difference.
What about the future? Say we want to move Drupal to be OOP. I guess the pain is already there since arrays are all over the place anyways.