My issue with "behemoth" comes into play when you're doing stuff like velociraptors or compsognathus or other things that aren't exactly behemoth in size, but things you'd certainly call "dinosaurs." And if you're calling something a "behemoth," there's no obvious reason you'd lump in T-rexes and Triceratops, but not dragons or Zaratans or hippopotami or Rocs or whatever. But it'd be a solid name for, say, sauropods.
"Tyrants" (T-rexes and similar)
"Behemoths" (Apatasaurus and similar)
...maybe some others.
Well, a lot of the grouping of things in the MM is done for the convenience of the GM, not necessarily as part of some world-building exercise implying that people in-game group creatures in the same way.
That's why, for instance, dire animals are included as a group even though biologically speaking a dire eagle doesn't have much in common with a dire bear, or a dire shark. And in-universe, why would everyone somehow recognize all these disparate creatures as belonging to some single "dire animal" category? After all, we don't call big bears dire bears, we call them kodiak and polar bears. Great white sharks aren't called dire sharks. Dire wolves existed, but that only drives home the point since they were the only animals to be so named, and in fact inspired the whole dire animal phenomenon in D&D in the first place.
Doesn't bother me though, because the MM is organized for the people who play the game, not the people who live in it.
Now despite all that, I think I could sign on to the idea of dividing dinosaurs into "Tyrants", "Behemoths", "Raptors", or similar categories. Few enough that it's still easy to reference them as a GM, but a little implicit world building baked into the MM (that is, as usual, easy to ignore or rearrange) would be welcome.
EDIT: Also, apropos of nothing, a dire T-rex or triceratops would be pretty sweet.