But this particular trend started from an effort to push things client-side for good reasons that are vital, or at least useful, to many projects. It being trendy doesn't necessarily mean it is wrong! Don't be so fast to dismiss something because to don't immediately see/understand the benefit.
There is often some confusion between "mobile first" and "offline first" - the two overlap considerably but being a good mobile app doesn't absolutely necessitate being able to operate offline and offline capability does not require or imply an app will be usable on mobile (it may be to large and/or fiddly in terms of UI on a small screen, not practical to use in a touch manner, or too demanding of CPU and memory resources). Some of the hand-wavey-ness is people who know they are driving for "mobile first" and think that means they have to drive for "offline first" without really understanding the benefits & implications of that.