I'm always amused at all the sub-genre Metal (Black, Doom, Post, Viking, Grindcore, Brutal, Star Wars, etc). It's all the same, no? The music features Cookie Monster on lead vocals, screaming lyrics that can't even be understood. Ok, all that being said, I'm a fan of Children of Bodom and Nevermore.
Also a prog/symphonic fan. Rush is my all time favourite, but there are so many good ones. I recently got into Sabaton (Sweden) and Trivium (USA), they are excellent.
It's like any other genre, it depends on how much of a concrete type of music you listen to and how deep you dive into it. But that sentence would equivalent as to say classical music is all the same, just a bunch of strings and classic instruments mixed all together, so why call this baroque and that romantic or just soundtrack music, it's all just violins, isn't it?
If you're not into a concrete genre you cannot hear the differences between the different subgenera but if you are into it and listen to a lot of bands the differences are very clear and you will see how Suffocation do not sound like Emperor or Judas Priest or Paradise Lost or Veil of Maya or Mötley Crüe... to just name a few examples.
I used to listen to a lot of metal in the past myself and rarely listen to it these days but I don't think it is childish or fake or anything like that, there are a lot of talented musicians in that genre that aren't dumb nor fake, but for me it just got boring, because even music with guitars and traditional instruments kind of got boring in general.
It is very hard to find interesting compositions that sound fresh these days so I guess I just took a different pace and started to add "new sounds" or new ways to mix traditional and new sounds in music in an interesting way.
Metal fans, like many other music genres fans, have a tendency to become very conservative and narrow the possibilities of artists and start saying this or that band is not true metal. The positive effect is that if you like that music you know where to look after bands you might like but the negative one is that it kills a lot of creativity or just pushes away bands that might be talented but don't match their expectations of true metal to 100% anymore.
At the end if you get bored with this style, what most people do the sooner or the later, that limitations can end up killing the whole genre and convert it into a niche for a very small public that will surely feel very good about being true to the genre but honestly, keeping it from advancing into more interesting ideas.
Anyways, I think metal is one of the genres that has survived this the best and that's actually because of those many subgenres that keep this style moving forward even when slower than maybe other music styles.