While I understand your point of responsive design, I feel we shouldn't be constrained by mobile first, and have to design for iPad (Portrait and Landscape) and desktop (usually Landscape only) as well
Mobile first shouldn't be viewed as a constraint. It's merely a starting point. You mentioned yourself bundle size and "It must never fail on mobile." An easy way to recognize how it helps is to compare to a similar (though not completely overlapping) approach:
While you may need elements of all paradigms on both sides of the comparison, mobile-first and progressive enhancement are easier to implement because you're adding what you need as you go rather than subtracting (which often ends up being more like replacing than actual subtraction).
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While I understand your point of responsive design, I feel we shouldn't be constrained by mobile first, and have to design for iPad (Portrait and Landscape) and desktop (usually Landscape only) as well
Mobile first shouldn't be viewed as a constraint. It's merely a starting point. You mentioned yourself bundle size and "It must never fail on mobile." An easy way to recognize how it helps is to compare to a similar (though not completely overlapping) approach:
Mobile-first -> progressive enhancement
Desktop-first -> graceful degradation
While you may need elements of all paradigms on both sides of the comparison, mobile-first and progressive enhancement are easier to implement because you're adding what you need as you go rather than subtracting (which often ends up being more like replacing than actual subtraction).