In "Living Witness", or at least those parts set in the future - in the 31st and ?33rd century - we see a planet in the Delta Quadrant that had no contact with the Federation since Voyager left. The lack of any Federation presence seem a bit strange to me
Perhaps this planet simply doesn't want to be in the Federation. They have that right.
Or, OTOH, the constant squabbling of the Kyrians vs. Vaskans would have disqualified them for membership, anyway (the Federation requires a one-world government). In the closing scenes, when the two groups are at peace, then they'd be eligible, assuming they have achieved political unity.
In any case, it's a huge galaxy. There's room for many different federations. As for *the* Federation, I find it very likely that they will exist for centuries, even millennia.