I see what you mean. I'm not really fully satisfied with any of the current tagging approaches and software support either. With footways it's a bit more tricky than with others, since the distinction between routable linearly vs. being routable as area is often a bit more blurry.

The OSM wiki does mention highway=footway+area=yes as an acceptable tagging approach and it is quite commonly used like that (there are over 54'000 instances on taginfo), though there is no clear consensus and some consider it tagging for the renderer. I'm not sure how much of an issue the current tagging is. Not all iD warnings need be followed, in my opinion.

Elsewhere I've done something slightly different (and I've seen others do it either way, too). Here the footway=traffic_island is also refining the area:highway=footway (as you might do for, e.g. area:highway=service, service=driveway)

area:highway=footway
footway=traffic_island
+
highway=footway
area=yes

Would you find this more acceptable? It's a bit more logical at least.

With area:highway not being so widespread yet, I think one could justify mapping footway areas with area=yes as well.