Now most of the internet is hinternet, and we're all forced more and more to rely on it. Banking systems, mortgage platforms, car payments, utilities payments are generally designed mobile first, desktop later, they employ various dark techniques for "verifying real user", which break on open platforms, forcing you to access them from iPads and other such locked down devices, or not at all. If hinternet used to be the dark shady streets where hucksters were peddling you knockoff watches, then now hinternet is the dystopian landscape of vertical information integration, ran, behind the scenes, by para-governmental institutions. You can't log in into irs without using id.me, a digital wallet and identity management platform, that sells you things.
There are attempts to cultivate little gardens of sophistication, but they are of mixed success. On a personal level there's a strong disincentive to participate in the hinternet beyond the mandatory, carefully navigating poorly designed and conceived systems just long enough to achieve an objective. One has to login into irs, but one doesn't really need to read that popup and upsell blocked, mobile centric news article.
From this perspective "mobile-first web design" is a symptom removed from its greater context.