I think the implied argument is "you can't safely assume that any market niche will be filled when the big player(s) go away, sometimes the niche just stays empty"
Digital cameras are not a niche. They constitute almost the entire global camera market. There are other review sites, forums, print magazines, etc. One website going away is not going to obliterate the world's ability to review one of the most popular consumer products.
You seem to keep missing the point that many people consider DPreview to be the best site in its niche by far. There is no certainty for its users that the impending competitive opportunity will yield a new option of equal or better quality. Mediocrity often outperforms quality in the market.
Camera sales, which are overwhelming which is overwhelmingly digital cameras, fell 93% from 2010 to 2021. In 2022 8 million digital cameras with a value of $5 billion were shipped.
Based upon those numbers I would say digital cameras are a niche and no longer one of the most popular consumer products.
