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I have no doubt that there are good use cases, your included.

The two issues are that it would complicate the snot out of
parsing (not a good thing) and that allowing keywords in
non-keyword roles would throw-off many existing tools (esp.
syntax highlighters).  

In addition, human eye parsing also suffers when keywords
can be used in non-keyword roles.  IOW, readability suffers.

For your app, it is a reasonable solution to wrap your code
with in preprocessing that converts keywords to non-keywords:

    s = s.replace('class=', '_class=')