I see this thrown around a lot, and have often seen it aimed in my direction, and it always comes off as sour grapes to me.
For me, a success stan is someone who will ONLY show interest in groups that are successful, and their allegiance will shift to whatever group is perceived as the top.
If you just celebrate the successes of your group, I don't see an issue. It's all part of the story and journey of an artist, and bringing awareness and spotlighting historical achievements is important. That's why they get national and sometimes international headlines, depending on the scale of the achievement.
Speaking personally, I laugh anytime someone calls me, a MooMoo of over a decade, a success stan. I've ADORed and followed Mamamoo since 2014, when they were mostly nugu, and I went to 4 stops on their tour in 2023, years after their last #1 hit song on any chart.
And even if NewJeans suddenly fell off in 2024, my NJ topics wouldn't.