They're still relevant because all of the members are active in the entertainment industry, be it singing, acting, MCing or variety guesting.
Aside from their individual talent, they utilise the brand SNSD to their advantage. Taeyeon who is a successful singer on her own, still say that she is representing SNSD with her solo activities. Yoona a successful actress, still respond with 'I'm SNSD' when being praised.
It also helps that SNSD members in general still interact very well with one another, supported one another during their individual activities. So yeah they're a good example why I do believe it's an advantage to not disband, but just have hiatus and have the option for a comeback open when the timing is ideal.