Pokemon such as Snorlax have "Stabilized" onto a single Evolutionary stage,
which results in Snorlax only laying Eggs that will Hatch into Snorlax.
But Snorlax still retains the DNA of it's pre-evolution Munchlax
Which is how it can be brought out and causes Snorlax to lay a Munchlax egg.
If we project this onto a Pokemon to potentially follow the same pattern
It's possible that Gyarados may one day phase-out it's Magikarp Stage
resulting in Gyarados becoming a 1-stage Pokemon and Magikarp becoming extinct.
Of course, much would have to happen to cause this result, including
(but not limited to) Gyarados becoming the Dominate evolutionary stage
as well as Magikarp becoming far fewer in number. So, not likely to happen.
I can agree with you to an extent. But imagine how the pokedex would be. It would be grass, fire, water for six cycles. Kind of would be weird.
In that dex, I would see the flying natives being moved next to each other.