ばかりだ(Only) continue to, Keep on -ing, More and more

ばかりだ attaches to the dictionary (non-past) form of a verb and is used to show that an action, often a negative one, keeps happening or is the only thing happening. In contrast, てくる combines the て-form of a verb with くる to describe an action that began in the past and has continued to the present. Unlike ばかりだ, てくる can be used for both positive and negative situations.

つづける and ばかりだ both show continuation, but with different nuances. つづける attaches to verb stems and shows simple ongoing action, e.g. 減り続けている 'is continuing to decrease'. ばかりだ adds the sense of 'more and more,' often negatively, e.g. 減るばかりだ 'keeps on decreasing'.

ばかり means 'only' or 'just', often suggesting repetition or excess, e.g. 文句ばかり言っている ('He does nothing but complain.'). ばかりだ builds on that to express that a situation continues to worsen or progress in one direction, e.g. 状況は悪くなるばかりだ ('The situation keeps getting worse.').

ばかりだ and 一方だ are similar in that both express continuous or progressive change. However, ばかりだ is typically used with negative developments and often carries a more subjective tone, e.g. 状況は悪くなるばかりだ ('The situation keeps getting worse'). 一方だ, on the other hand, sounds more neutral or objective, and doesn't necessarily imply a negative trend, e.g. 人口は増える一方だ ('The population keeps increasing').