这个优势地位逐渐随着其他国家的日益强盛而缩小是不可避免的.
这是should have done的用法：
请看下面的资料：
should have done只用于谈论过去情况,主要有两个用法：
■用于推测过去已经发生的情况.如：
He should have arrived by now.此时他本该到了.
The letter should have arrived by now.信现在该到了.
The train should have already left.火车大概已经开走了吧.
I can’t think why he should have been so angry.我想不通他怎么会这样生气.
■用于指本该发生而实际上未发生的情况.如：
You should have told me so before.你早就应该告诉我.
Look at the time!We should have been at the theatre ten minutes ago.瞧都什么时候了!十分钟前我们就该到戏院了.