A slower growth trend would be "natural" as you describe it, but a drop in users should only be concerning at this stage, especially as the platform is still so young. Even a small amount of growth is still growth but a decline in users means more people are leaving the platform than joining it.
Again, you're pulling explanations out of thin air - go ahead and prove that those users are switching to kbin over lemmy, use some data to back up your claim.
Or accept that we have a problem with adoption and as a community we need to fix it.