I think the term "rip off", as you used, is the wrong term here. A rip off, in this context, refers to someone(thing) directly stealing an idea from another source, which I don't think is the case. For one, it's all within the same series - I think Wolf Sage put it best when referring to a throwback or simply a reference.
However, I don't think it comes down to just a reference, I think it's key that the series maintains some level of consistency between games and such. A better word would probably be "continuity" and this creates the illusion of it - seeing the likes of Armogohma in Twilight Princess made me straight away think of its Ocarina of Time counterpart, Gohma. Then you can give a certain level of importance of said boss, or variations. It doesn't just have sentimental value but also value of the lore itself.