Dear Pankaj
Thank you for your comments.
I am sorry if you misunderstood my intention.
I did not try to argue which was more fundamental: physics or mathematics. Physics is a description of the natural world. Mathematics is a language that describes things in two very different ways: relationships such as symmetries and quantification with numbers.
I worked to show, explicitly, that the concept of consistent quantification constrained by symmetries leads to specific mathematical laws. (in the example given, it was additivity)
You seem to be claiming that quantum mechanics may not be governed by symmetries and thus not quantifiable via mathematics. Don't give up too fast! We have shown that the Feynman rules for manipulating quantum amplitudes is indeed derivable from fundamental symmetries:
Origin of complex quantum amplitudes and Feynman's rules
Philip Goyal, Kevin H. Knuth, and John Skilling
Phys. Rev. A 81, 022109 - Published 11 February 2010
http://journals.aps.org/pra/abstract/10.1103/PhysRevA.81.022109
and
Quantum Theory and Probability Theory: Their Relationship and Origin in Symmetry
Philip Goyal, Kevin H. Knuth
Symmetry 2011, 3(2), 171-206; doi:10.3390/sym3020171
http://www.mdpi.com/2073-8994/3/2/171
These are proofs---not arguments.
Indeed QM *can* be derived and from consistent quantification constrained by symmetries!