That's a bit of a jump. You go from "mechanical systems are patentable" + "some software emulates mechanical systems (which are patentable)" to "all software, regardless of whether it is related to a real mechanical system, is patentable".

Does that not seem like a huge leap? mpeg2 sure isn't encoding a mechanical mechanism. Hardly any software these days really is. Software now is mostly focussed on dealing with ætherial information, not physical machines.