That's all it ever was.中all与it是不是同位语？

应该改为下面的定语从句。在从句充当表语成分。

That is all that it ever was.(关系代词that 代替先行词 all ）

=That is what it ever was.

The Cosmos is all that is, or ever was, or ever will be. Our contemplations of the Cosmos stir us.

宇宙是所有现在是的，曾经有的，和未来会出现。我们对宇宙的冥想令我们激动不安。

Ultimately, that is all that you can ever do.

基本上，这就是你所能做的一切了。

That is all that there is to it.

不过如此。

That is all that I have to say for now.

这些就是现在我所要讲的。

The girl is unique, and that is all that I can say.

这姑娘天底下少有，这就是我能说的。

That is all that I can tell you.

=That is what I  can tell you.

那是所有我能告诉你的事情。