We believe that this is only an inevitable trend, but one that will prove beneficial.

but 是表转折的并列连词，必须有，不可省略，以表示前后两个分句间的转折、对比关系。本句but 之后省略了it is，原句等于：

We believe that this is only an inevitable trend, but it is one that will prove beneficial.

这里one 仍然是泛指，即便有了定语从句修饰也是泛指。

We believe that this is only an inevitable trend, but it is one that will prove beneficial.

【翻译】我们认为，这是一个不可避免的发展趋势，但它将证明是有益的（一个趋势）。