一方だ: a trend continuing in one direction
Use 一方だ for a continuing increase or decrease. Distinguish repeated actions from directional change, check the limits of evaluation and prediction, and compare 一方で.
Which direction is the change taking?
この地域では空き家が増える一方です。 says that vacant houses in this area keep increasing. 一方だ follows the dictionary form of the change verb 増える to emphasize the trend's direction.
参加者は減る一方でした。 describes participant numbers continuing to decrease in the past. The preceding verb sets the direction, so do not memorize 一方だ as an expression only for increases.
A trend rather than a repeated activity
毎日この店で昼食を食べます。 describes a repeated lunch habit. Instead of turning simple repetition into 食べる一方だ, check what quantity or condition is increasing, decreasing, or otherwise changing.
仕事が忙しくなる一方です。 says that work keeps getting busier. なる supplies the change, making the developing trend more prominent than a repeated state such as 毎日忙しい.
Negative evaluations and the future
The expression often fits unwelcome situations such as 悪くなる一方です。, but does not itself mean something bad. 二人の親しさは増す一方です。 can describe two people's closeness continuing to grow.
A present 一方です does not guarantee that the same direction will continue forever. Unlike 一方で linking another aspect, 一方だ rounds off a statement about one trend.
Quick check: Does 不安は小さくなる一方でした。 mean that anxiety kept growing?
Show the answer and why
No. It says anxiety kept becoming smaller. 小さくなる sets the decreasing direction, and でした places the trend in the past.