Phones moved quickly from HD to QHD and even 4K panels. For most users the extra pixels are invisible, while the battery cost is real. Today the bigger story is adaptive refresh rates up to 120 Hz, HDR and OLED efficiency.
For app teams the lesson is the same: design with density-independent units, supply vector or multi-density assets, and test across screen sizes and foldables rather than chasing a resolution number. Smooth animation and fast loading matter more than pixel counts.