Hi Prody. I'm always excited to see an English-speaking Nico user!
You're right that view count is way down. Technology has evolved, and both creators and viewers have many other options now. But aside from things like Inmu that have their own culture, haven't the source clips in most popular 音MADs always been the current trend, even in Nico's early days? Most popular trends since have their own several 音MAD masterpieces, so I don't think much has changed in that sense. There will always be new source clips, and I see enough new 音MADs that I'm not too worried about their viability as a genre going forward.
In any case, the guide is for people who don't know much about Nico. Given the popularity of YTP and the few non-Japanese 音MADers (as recent as OctagonCollaboration), there's definitely a non-trivial number of people who would enjoy 音MADs but don't even know what's out there. I'm sure you can see the value of an organized list of tag translations to them, even if nothing new were to ever come out again. In fact, I'm pretty surprised that this doesn't seem to have been done before. It's nothing hard.
I'm curious. Do you know of any non-Japanese online community of 音MAD fans? Is some variation of "Oto MAD" the standard translation? Is it even discussed anywhere with enough frequency to be called a standard?