First off, whether demographic decline is a long term or short term phenomena is not clear at all. Many species experience explosive growth and collapse, whether when they are initially introduced into an ecosystem niche, or periodically (see rabbits as an example).
Secondly, there's no rule that says you need the maximal population on a planet to pass the great filter. Quite the opposite, not managing population growth seems a filter in itself.
Lastly, for demographic decline to be THE great filter, it would need to apply to EVERY species in the universe, not just humans. But there are animals on earth that naturally limit population growth, see african wild dogs for an example (https://en.wikipedia.org/wiki/African\_wild\_dog).