The “branches” are not churches. The branches are people. The Vine (Christ) is the Church. And as there is only one Christ, there is only one Church.
That's fine. We're on the same page there (Greek ecclesia = "called-out ONES" ... flesh & blood...not brick & mortar or hierarchical bureaucracies)
If you re-read the Mormon "first vision" it proclaimed that ALL (as in 100%) of the professing believers at the time of Joseph Smith's pronouncement were "corrupt"...
Yeah, we're all sinners...but Smith practiced a "scorched earth" policy in order to try to make the need for a "from scratch" need for an entirely new church.