Why not allow the array form of $cond to accept longer arrays of if-then-elseif-then conditions? Instead of restricting it to only accepting arrays of length 3, it could accept any array of length 2*n + 1, with obvious interpretation:

{ "$cond": [ if, then, elseif, then, elseif, then, ..., else ] }

. This is clearer and simpler than having to nest multiple $cond terms.