This matches my observations pretty closely. A while back I ran some comparative historical numbers for 20XX and 19XX to look at two things: (1) are the popular characters in 19XX more or less popular than the popular characters in 20XX, and (2) are the successful characters in 19XX more or less successful than the successful characters in 20XX.
The answer to both questions was 'yes'. Setsuki in 19XX is more popular than Zane or Troq in 20XX, and more dominant than either of them in 20XX, if I'm remembering correctly. The Argagarg numbers you're showing, probably don't reflect the fact that players now expect a higher number of strong pocket Glorias from their opponents than they would have in 19XX 1?
In one sense it doesn't bother me because one of the reasons for the format was just to see less Zane, Troq, Geiger, and DeGrey. Which it would be very hard for 19XX not to succeed at. On the other hand, I had also hoped to see a "wider" carousel develop: instead of "Top 3 and Friends" I hoped for "Top 6 and Friends", or something.
I haven't entirely given up on that happening, actually, and part of the reason is that historical results, and predictions based on historical results, don't account for discovery and personal growth. I feel like up until about a year ago, BBB was still considered a hard counter to Troq, and I don't think that's the consensus now. And BBB/Troq isn't what I would call a very complicated MU.
I think 19XX right now is still at the "I'll play my main OR Setsuki/Argagarg if it's a good MU for them OR a pocket character that I have against some specific character." I don't think that people are really experienced with "19XX characters that do well against Setsuki" yet, and I still think there's possible room for an unexpected CP against Setsuki to arise out of the restricted options in 19XX.
Maybe that won't be borne out, but I don't think the nail's in the coffin yet. I think Setsuki and Arg still benefit a lot from being relatively easy to play for their power level in the meta - very similarly to Zane and Troq in 20XX. So there may be answers to Setsuki "at character select", but they're not as easy to play (or specifically play against Setsuki), and so she still enjoys a significant advantage in the meta.
...or I'm wrong and she's busted because Troq/Geiger aren't available to CP her. shrugs Right now I still narrowly feel like it's the former rather than the latter.