Yes, that's true. However, the branches in the bloodlines could have come after Leon. Leon could have given birth to say... three children, and each one of those children had multiple children, anywhere from two to three. At that point, even though there's so many, they're all directly related to Leon.
The bloodlines deviating is evident, particularly in OOE. But we know to some extent, the main bloodline (presumably from Leon) was an important part of the Belmont lineage. Simon is referred to as Juste's grandfather, the GBA titles are about father/son Christopher and Soleiyu, and even Richter refers back to his Ancestor, Trevor. Julius is presumably part of the original bloodline.
When I say direct descendant this is what I mean:
As you say, Leon Belmont may have had 3 children who have 3 children, who have 3 children etc, and this multiplies the Belmont family, all being related to Leon.
However, Two things are apparent:
1) There will always be a Belmont heir who is male. We know this because the name 'Belmont' has spanned from Leon>>>>Julius, over 1000 years. What we can assume from this is what a male will always be born in the Belmont bloodline.
The name was never lost, hence, there was always a male to carry that name, in every generation. All we can assume is that if there is more than 1 male, the most worthy heir inherits the VK. For some reason the heir is never female. (they haven't yet been)
2) We know that there were instances when females were born and the name was not carried (Daniela; OOE - has memories of fighting monsters with her grandfather) Canonically, Bloodlines and POR are the only instances when Belmonts who didn't carry the Belmont name, wield the VK. At some point after 1797, the Belmonts no longer use the VK and the last Belmont to use it is Julius.
Again the users of the VK are male, whether this is coincidence with the Morris family or not, is debatable.
My point is that just because someone is directly descended, it doesn't keep the desired bloodline, nor does it maintain the family name.