( )has been said above ,grammar is a set of dead rules.

因为这是个定语从句，as的用法在此相当于which，也就是说
He was beaten in the game, which has been expected.
他在游戏中以失败告终，这是预料中的事，答案中的as就相当于which
What一般都是用于宾语从句的，例如：I don't know what you are talking about.我不明白你在说什么，what you are talking about构成了一个宾语，在句中形成了一个宾语从句。
如果你要用what来转换上面那句子的话，可以这么说：
He was beaten in the game, and it was what has been expected.
或者What has been expected was that he was beaten in the game.

如果用what，则后面的为一个短语，而这个短语与前半句的名词不构成同位语，没有相关性，语法上不成立。如果用what，可以这么说what has been expected is that he was beaten in the game.意思一样。