I agree, well explained.I disagree.
You're portraying this situation as if Diglett/Wigglet and Meowth/Alolan/Galarian Variant are the same situation. They are not. That's the whole point. All the Meowths have a common ancestor and *are* related in that way.
Wigglet and Diglett do not, they merely resemble each other.
Divergent = species with a common ancestor that change to become increasingly different over time (Meowth/Galarian Meowth/Alolan Meowth)
Convergent = unrelated species that develop similar characteristics over time (Diglett/Wigglet)
As for the names of Pokémon, it is probable that the humans have named all Pokémon species through history and time. Similar to how in real life, humans are the ones who have given each animal / creature a name. Because Wiglett and Diglett look very much alike, the concept is probably that the humans in the Pokémon world have also named the two different species with very similar names due to their very similar striking appearances.
I think we have to try and step out from the knowledge and awareness that Nintendo named these Pokémon and put ourselves into an imaginary role as if we are within the Pokémon world to understand why fictional people have given two Pokémon that look almost identical very similar names. Kind of like:
'Oh this Pokémon looks very much like Diglett but it really is a different species of Pokémon. For this reason we shall call this species Wiglett'