Firing Rierden when they were slumping in January and February when they were still a top 2-3 team in the league would have been a rash decision. They did the right thing and let the team see if they could play through it. This team has seemingly always been in the doldrums during the February portion of their schedule, especially when the playoffs were all but a statistical certainty, and they come through it with a vengeance in March. Even during Oates' first year they finished the season something like 15-2-1 after looking like they'd miss the playoffs in Jan/Feb. Trotz was almost fired in February in 2018.
Them struggling through this period is their norm and not the exception, but not being able to play through it and somehow getting worse is not. What's more worrisome is that there are no indications that the team is close to finding their way out of it with 15 games remaining and they're panicking, which is why pulling the trigger on Rierden at this point wouldn't be surprising.