Means that something continues changing value (verb)ing.
It gives a feeling that the person saying it is an impartial observer.
このままだと、彼女の 収入が減る一方だ。 このままだと、彼女の 収入が減ばかりだ。○ If this stays as it is, her income is going to keep on decreasing.
Unlike ばかりだ it can be positive, or negative.
お客さんは増える一方だったはず。 お客さんは増える一方~~ばかり~~はず。✕ The number of patients would have kept on increasing
アメリカとロシアの関係は悪化する一方だ。 The relationship between the US and Russia keeps on getting worse.