In some countries now (Poland is a good example), so many of the sidewalks are actually combined or segregated footways/cycleways. This means they have to be tagged separately using the relevant preset, so that bicycle routing is improved. Also, a sidewalk will possibly have different surface= and smoothness= values from the road that runs alongside it.

area:highway does already render in map apps in wide use like OSMAnd, and the area:highway tagging has long been considered mature already across multiple countries.

Keep in mind that the map at openstreetmap.org is just a tech demo, not a real consumer product. It lacks support for rendering many tags that have been well-established and widely used for a long time now.

You shouldn’t base your tagging choices on what produces the desired rendering on the openstreetmap.org map.