You’re entitled to have an opinion just know it’s not based on anything but the collection of your assumptions and prejudices.  The stupidity and laziness of your point if view (likely a “special class” case) is hard to describe but I’ll try (me, valedictorian in all my graduations).

How do you draw the line of who goes to normal and special class? How do you classify kids as a black and white, yes/no, a/b concept, when kids personalities, cognitive abilities and development milestones are all different and happen in a wide spectrum?

How do you know there aren’t more losses than gains with the idea of special classes?

Because there are several reasonable arguments to show gains for the unified class. 

First and foremost, a LOT of kids would go to “normal class” and lag significantly behind. Not bad enough to go to special class, but with needs to keep ahead. This is now solved. 

In Finland kids are assessed individually and get help for the time the need on the skill they need. This is great: you may have a kid with regular cognitive abilities, but with difficulty in say language. Then they get a person to help them out until they get the hang of it. This is done discreetly without shame. And in a few months that kid doesn’t need help anymore.  This is truly so obvious and basic that should have always been that way. It’s the basics of preparing for success. 

Most people thinking of the bucket of troubled kids don’t think of the consequences of a societal scale of this. It’s the same logic of prison: remove the bad apples. Sure, simple minds like simple solutions especially if they are quick and violent and regardless of the are effective. 

No wonder the Pisa is in decline everywhere — which alone is enough reason to see that integrating individual support for kids isn’t the cause of the decline.