Is this sentence grammatically correct?

Question

While studying ばかりだ I found in a grammar book the following example:

日本語の成績が下がるばかりだ。

It was also written that in this sentence ばかりだ cannot be substituted by 一方だ, but searching on the internet I found many sentences with 成績が下がる一方だ. Now, I'm not sure if I didn't understand the difference between ばかりだ and 一方だ, or I read the grammar book incorrectly. Can somebody clarify it to me?

Answer 1

IMHO both 日本語の成績が下がるばかりだ and 日本語の成績が下がる一方だ are perfectly fine because it's about an undesirable change.

• 一方だ only refers to a temporal change, whereas ばかりだ can refer to a bad thing happening many times (≒"always").

  × 迷惑をかける一方で申し訳ない。
  ○ 迷惑をかけるばかりで申し訳ない。

• ばかり almost exclusively describes an undesirable change, whereas 一方だ can also refer to a good change.

  ○ 彼の日本語の成績は上がる一方だ。
  × 彼の日本語の成績は上がるばかりだ。

Answer 2

I don’t know the Japanese grammar correctly. but as a native Japanese speaker, 成績が下がる一方だ is very natural.
I also search internet , most japanese article says it’s correct.