tl;dr the data published in the Original Post does not (by itself) support the hypothesis that participation is declining. The number of participants in "this month's" content is always less, for predictable reasons.
The ideal comparison is to use "snapshots" for participation after 2 months. Since we don't have that, the next best is comparing relative participation and that shows a different picture: relative participation is similar.
(My personal belief is that the OP is likely correct that participation is dropping for metas. But I don't think we players have access to any data that can demonstrate that. Instead, I think the most effective way to convey "meta fatigue" is to post our opinion, rather than trying to teach ANet what their numbers mean.)
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##### In More Detail
>! The original data doesn't show that the participation numbers increase as time goes by. The numbers for the most recent content will always be less than for last year's content, even if the game's population has dropped. The current episode only has the early adopter participation; the oldest episodes have all the new, returning, and "didn't feel like it" participants.
>!
>! A true apples:apples comparison requires taking a snapshot of completion rates at various times, which GW2 Efficiency doesn't report on the main site. It's probably okay to compare 4-year old achievements with 3-year old ones; as time goes by, the cumulative differences will diminish. But it's definitely not reliable to compare 3-month old achievements with 6-month old ones.
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##### Proxies for Comparison
>! One proxy I've used in the past is to compare relative completion rates. How many started versus how many finished? Using the OP's numbers, those ratios look like this:
>! * Bound by Blood 0.82
>! * Whisper in the Dark 0.91
>! * Shadow in the Ice 0.79
>! * Steel and Fire 0.95
>! * No Quarter 0.75
>! * Jormag Rising 0.82
>!
>! Notice that *Jormag Rising* is the same as *Bound by Blood*, and *No Quarter* is close with *Whisper in the Dark*. These rates go up and down, partly because not everyone enjoys each episode as much as the next, partly because Real Life™ affects us different in each month, and, as some noted above, the ability to gain other rewards is sometimes gated by story completion, and sometimes not.