I mean it took Roger who
- Defeated Rocks, the strongest pirate crew in the world
- Defeated WB
- Fought and defeated (and lost to) Garp, the strongest Marine alive at that time, multiple times,
- Became the PK, found out the true history
- Possesed an ancient Weapons
And so on
.. to even crack 5 bil
It doesn't make that much sense for Luffy to immediately jump to 5 bil by "only" defeating Kaido.
And that's good, it protects WB and Roger, as well as Kaido + Big Mom's statues as legends, as well as give some room for Luffy to improve.