A 600mm F/4 lens gathers a lot of light per-unit sensor-area, and there are many reasons why professional nature photographers have historically needed such a large lens

(1) Older digital sensors weren't good at all with high-iso (high read-noise, low photon-electron conversion efficiency, etc)

(2) An off-sensor DSLR AutoFocus array has a very small effective-aperture, making the lens aperture very important in low-light situations for autofocus (which a lot of nature photography is)

(3) Bokeh - Ofcource nothing beats large aperture bokeh, but this can be tackled perhaps through computation?

For (1) and (2) Modern imaging sensors perform beautifully in high-iso scenarios. Also, with mirrorless, there are cameras that can AF down to F/22 - something unimaginable (and impossible) with DSLRs.

While the demand for 600mm F/4 is there from a professional standpoint, there is an even larger demand for cheaper options - and.. this point is important, you can produce award-worthy images using those cheaper potions. In fact if you look at any nature competition a lot of the awards go to images made with cheap/budget gear.