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  > How about making the byte compiler recognize the construct

  >  (if (< emacs-major-version NUMBER) ...)

  > and do this optimization on it?

People seem not to have considered this seriously, but I have not seen
any serious discussion of a drawback.  What flaw or drawback do people
see in it?  It should optimize the existing the existing code with no
change at all.  Isn't that just perfect?