If there was no dominant or common product in the industry, then I would agree. But if anything is commonly sold, then that disqualifies itself as a niche product. I would never consider term life insurance as a niche product as it is the most commonly sold item. By definition, high sales and low cost item in any industry is a mass market item, not a niche market item.The way you phrased your question I don't think you understand what the word niche means.
Niche means it's a specialized segment of the market. So yes annuities is a niche. IUL is a niche. Term insurance is a niche. None of these things is the entire insurance industry. They are all a specialized portion of it.
I have never heard of anyone mention term life as a specialized product. It is the most basic, the most general life insurance product.