There was a concerted effort some time back to formalize a standard measurement of process density using something like "million transistors per square centimetre" by a standards organization. (IEEE?) It wasn't a perfect measurement but it was a lot better than width. It failed so completely that I can't even Google it any more. The awkward name probably didn't help.
edit: it's "MT/mm2". Some people actually use it, but more in the informal sense that has the problem you espoused rather than the formalized one, which I still can't find.
We can always increase W, but decreasing L requires technological advancements
So I’m not sure on the value of area (W and L), L alone is more relevant (for the reason I said above)
we are dealing with finfets / GAA now which are not the same as planner transistor, but I guess there should still be relevance in L and W because these measurements are important even for resistors
So probably some sort of equivalence between a FF and a planar transistor should be given to name the nodes