As it is currently designed, there is zero reason for the Gnashing Fang combo to not be one single button. It's not a "real" second combo since it has a CD and you can't mess the order up at all. Other combos in the game can be screwed up by pressing the wrong buttons or can be used twice in a row, leading to a wrong rotation (like doing Chaos Thrust twice in a row for example), but this one is basically just like an IR/Delirium/Requiescat spam window but instead of pressing the same button over and over you press 3 different buttons with an oGCD spam in between. It's artificial "complexity" to make you believe that you're playing a complex job when you're actually just pressing things on CD all day (same goes with Sonic Break being on its own CD instead of being a DoT to refresh and actually watch for).

I believe that all combos in the game are unnecessary button bloat. You're basically using 3 GCDs and 3 hotbar slots for just one complete action/effect. Other games have way better combo systems, like ArcheAge for example, where every skill is a stand alone skill with its own effect, but you can combo some with others leading into extra bonus effects. And you have plenty of them possible between skills from different skill trees. In FFXIV, Fast Blade and Riot Blade for example are completely useless on their own. Their only purpose is to lead into Goring Blade and Royal Authority. They could just make all of them single skills and add in a lot of other skills to create much more interesing mechanics and rotations for every job. But that would require an entire rework/overhaul of every single job in the game and the overall battle system. I think the best solution for them would be to condensate all combos into single buttons (or make combinations of two buttons, like 1-1-1 being one combo, then 2-2-2, then 1-2-1, then 2-1-1, etc... to create even more combos while still using only 2 buttons), that way they have the room to add these interesting extra mechanics to every job without having to completely rework them.