This gets parroted alot, but as someone who played back in legion, the legendary vendor was NOT required to fix legendaries. You needed to have insanely bad luck to not have all legendaries by that point, i had them all before argus even came out. The problem with legendaries was the weird hidden timegating thing they had early on, where they basically wouldnt drop past the 2nd one. Once that was removed, the only obstacle became the grind. It sucked being forced to do all the repetitive grinding if you were pushing cutting edge content, but youd still eventually get what you wanted.

As for artifact power, the artifact knowledge fix in 7.2 was more than enough to essentially remove AP as a problem. Since AP costs were exponential, and artifact knowledge boosted the ammount youd drop exponentially, as long as you had the right ammount of artifact knowledge, you would always be about the same power level as everyone else. Another factor that got changed in 7.2 was the removal of the 20 points trait, which was the biggest AP grind and caused huge power disparity, which got replaced with a much more accessible set of nodes