I'm sure a good case can be made for culture progression. The ones who made the rules, and possibly still make up a large, if not majority, portion of the site's audience, are those who grew up in a world where American animation was not keen on taking visual cues from Japanese animation, as evidenced by the fact that Japanese animation is called precisely that
. Some - like Astro Boy, Gigantor, and Speed Racer, possibly Battle of the Planets or its G-Force reincarnation - may have been passed off as simple cartoons in the early days, but by the mid 90s (or even earlier, depending on your audience) it was often made a selling or trivia point that these shows came from Japan and were inherently different than
domestic fare. They stopped trying to hide it (shows being butchered for the wrong demographic notwithstanding), and the result is that country of origin is a contributing factor to how it gets classified. By that point, kids were smart enough to know that this cartoon that looks nothing like the rest of the stuff they watch was from Japan. Regardless of whether the distributors tried to hide that or if it was openly declared to be originally Japanese.
Following from that, they see Japanese animation as a distinct entity, namely, animation from Japan - and usually at the exclusion of outsourced stuff. Being inspired by something doesn't make it the same as. The concept
used to define the English usage of the term 'anime' developed long before the term 'anime' was adopted by the mainstream English fandom (when I was a kid, even up into the first part of middle school, it was far far far more common to hear it referred to as the full 'Japanese animation' term or as 'Japanimation'...only the Sci-Fi Channel used the term 'Anime' in their marketing for it, unless you asked someone involved in the early hardcore fandom - who would also never accept current anime-inspired Western animation as anime either). As far as I care to acknowledge, the only reason the back-loanword started getting used was for Western fans of such to identify with the cultural norms of the country that their shows came from, not to use the same base definition of the term that Japanese does.
I mean, if we're going to make arguments from animation style, styles change. There may be certain techniques that predominate during a certain period, but homogeneity is far from true these days. It was far from true then too. And if we want to use a superficial construction of what 'anime style' is (let's say, the 90s), SWAT Kats
looks far more anime-inspired than Avatar does, right down to the explosion smoke bursts and limited animation techniques. And it's from 1993. But Western fans are most certainly not going to call SWAT Kats anime, even if the influence is so obvious that it may as well hit you over the head with a hammer.