As someone who has watched since "super mario 64 bloopers: scatman's revenge", I can tell you that Luke's most recent video about "quitting" because he "did everything he wanted" is such bullshit.
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There never was meant to be some big overarching story in the first place. The channel was always meant to be just Mario doing random shit because he wanted to.
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All of these arcs and stories were just meant to be sprinkled in every now and then to be something refreshing from the constant randomness, instead of the other way around (which is why everyone loved the Waluigi arc, and why the other arcs were way more divisive). The channel began taking itself too seriously for one where a naked Mario humped spaghetti almost every other video.
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I can tell you this; Kevin becoming a writer is what started the slow-burn ruination of the channel, with his addition of too much anime, permadeaths, taking itself too seriously, etc.
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One of the worst things SMG4 did, was retconning all the old videos with the cosmology arc. A big middle finger to classic fans to basically say "oh all these funny old videos were because Mario was accidentally corrupted by SMG4 this whole time"!
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SMG4 was meant to be a place where Luke could make videos about random things he and his friends came up with. Mario gets his dick stuck in a door? Good idea! Mario scratches his ass? Good idea! Mario eats radioactive pasta that a doctor threw away in the trash and becomes a monster? Good idea!
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Back then, you could just start watching SMG4 from any video, and you wouldn't be missing out on some story. Now try showing a total newcomer "SMG4: World War Mario" without explaining the whole story first, and I bet they'd be totally confused.
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As someone who actually likes stories and seriousness somewhat, I can tell you that it would have been slightly more bearable if all his arc videos had a recap of the previous arc episodes to help any newcomers understand what was going on, and to make it obvious from the thumbnails that an episode was part of an arc, so that those who don't give a shit about those things, could skip it.