Wayland doesn't require you to use GPUs. Only wl_shm is in the core protocol, you can build a software-only Wayland compositor without any hacks, with shared memory.

By the way, Wayland has nothing to do with drawing text, lines and icons. You just give Wayland a bunch of pixels.

That said, using the GPU for compositing is often better for performance and battery savings. It doesn't require anything fancy, GLESv2 is used by many Wayland compositors and is widely available, even on old hardware. It doesn't require proprietary blobs.

If one turns off special effects (compositing) and uses low resolution, even the cheapest mainstream gpu is sufficient to draw 2D text, lines and icons with 60Hz framerate. However, people expect to get more in graphics output than 10-20 years ago, so they buy high-res displays or turn on additional graphics effects in graphics environment - and then, we do need powerful GPUs, because those resolutions and effects require high computing power that is best to provide in a powerful GPU, not powerful CPU.

Because 3D is just an extra dimension for the same calculations?

The GPU does not accelerate 3D, but the computations (so 3D can be much faster).

Because people want their desktop to look nice like Android with smooth animations and not like Windows 3.1 with grey rectangular windows and buttons.

As mentioned by others, you don't actually need a GPU. Weston, the reference compositor implementation, will run in software rendering.

To get that load off the CPU mostly.