Huh! Lol. 30's isn't old. I am the old one in my 50's and whether you like it or not, they are losing their grip. Their songs have declined in quality and as my daughter has said "they haven't had a decent song since 2010" and she would know as she was the one who got me into K pop in the first place. Lol to you, sorry.
For someone so "done" and "already moved on" from SJ, I'm really amused by the fact you kept posting on every forum related or somehow related to them. You kept informing or reminding everyone about you not being a fan of them anymore because their music is declining, these and that etc. The way you acted, it seems to me you are indenial and you are convincing yourself that you really not into them anymore. 🤣
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To answer the OP's question. Yes. Maybe they are loosing fame because there are new groups now; new trends; new style of music. It's inevitable. FAME is not forever But as long as they are passionate and is willing to perform and is profitable, and there are fans who still believe in them and show support. I don't think fame would matter anymore to them, LONGEVITY is.