Dear Martin,
I think you are over-interpreting what Ghislain wrote. What he said is
rather that your package will likely build unmodified on the next
release of Ubuntu, if not on the current one, because Ubuntu will also
upgrade there version of libav.
You don't *have* to go out of your way to ease the downstream work.
Likewise, you don't *have* to make any efforts to make it easier to
backport your package to older versions of Debian.
However, when it is practical, it is good practice to make the
adjustments that will make your package compile unmodified on Debian
stable and up-to-date derivative distributions.
On the other hand you don't want to obfuscate your package just for the
sake of backports. If the patch becomes unreadable, your sponsor will
give up on checking it.
In the end, it's up to you (and your sponsor) to decide the level of
complexity you want to allow in your package.
Kind regards, Thibaut.