There is no datetime.totimestamp because the range
of time represented by a datetime object far
exceeds the range of a normal int-based Unix
timestamp (roughly 1970-2038).  Datetime objects
before the start of the Unix epoch would be
represented by negative numbers.  As far as I
know, the common Unix library functions which
accept epoch times wouldn't know what to do
with a negative number.

That said, you stated these missing methods
were important.  Can you offer some use
cases which would support that contention?
I personally don't think a argument for
symmetry would be a convincing use case and
that's the only one I can think of.