Seems to me there's a simple question of practicality here. Making a class abstract takes away the programmer's ability to do something, namely, to instantiate it. There is nothing you can do with an abstract class that you cannot do with a concrete class. (Well, you can declare abstract functions in it, but in this case we have no need to have abstract functions.) So by making it concrete, you make it more flexible.
Of course if there was some active harm that was done by making it concrete, that "flexibility" would be a drawback. But I can't think of any active harm done by making Object instantiable. (Is "instantiable" a word? Whatever.) We could debate whether any given use that someone has made of a raw Object instance is a good idea. But even if you could convince me that every use that I have ever seen of a raw Object instance was a bad idea, that still wouldn't prove that there might not be good uses out there. So if it doesn't hurt anything, and it might help, even if we can't think of a way that it would actually help at the moment, why prohibit it?
