I get exactly what you mean though but for me, that is attributed to the 'lazy' culture of today who think that by taking someone else niche from a popular forum. In this instance, lets say ForumPromotion as it is one of the biggest 'promotional' sites I've seen anyway. I have seen so many sites pop up which have either copied sections, threads or section descriptions right from FP but then you have the idea of the niche itself. Unfortunately with forums today, most people do not see them as a community based area where they can sit and have a nice chat about whatever, they simply see them as money spinning operations.
Take a 'noob' for instance. 'Oh, ForumPromotion has nearly 900,000 posts which must mean this is an awesome niche and I must make a forum based on the same idea with the same stuff so that I can make uber $$$!' and you'll have to agree that this is the mindset of most people today.
I agree totally with your post and I think your points are very valid. There are quite simply too many and of course, with the saturation comes the problem of making your site stand out, making it unique. You can't stop people making sites though but I just wish that people would make a forum for the sake of having a forum, not just to make money. I mean, I can only imagine what places like FP or wherever get in terms of financials and probably, most other people wonder as well.
But hey, you can't stop people being lazy and thinking they can make a quick buck but then again maybe there are so many admin forums because a lot of people like admining, you never truly know. Anyway, I hope that ramble made sense, lol. I do agree with you though, 100%.