I started in patch 2.4 playing on my friend's laptop actually. I'm not going to claim to be some top player, but I cleared 13/13 mythic pre 7.0 and my logs are generally fairly competitive. Even have a few top 50 logs.
Anywho, the bolded, it's funny how far you've just stuck your foot into your mouth, because Slootbag literally said the exact opposite of that on his stream two nights. Imma go find the timestamp and link his drunken rant about how hard the game is, just for you.
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Found it! Starts at 9:50:00 when he starts talking about Gul'dan. He goes on to talk about other bosses a couple minutes later.
https://www.twitch.tv/slootbag/v/83408773
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First off, the bolded, that's only partially true. A lot of innovation gets done on bosses when they're live. Also, last phase Garrosh was untested. If I recall, Method actually thought they killed the boss the first time they got to P4.
Furthermore, gear will not affect every encounter the same way, so it absolutely matters if you had an extra 6 months of gear.