It really isn't that ambiguous in cursive, though. When writing out "dx" as product in cursive, the "d" and "x" characters are written separately. When writing "dx" as an infinitesimal, the characters are connected. Never had any issue with higher degree derivatives, either.

But I see how it can be confusing with printed characters. I guess Leibniz just took ligatures for granted when he came up with his stuff.