It wouldn't have been "natural progression" because why would Pichu, who has no apparent theme, evolve into Plusle/Minun, who have a Plus/Minus theme? Pokemon tend not to totally switch up like that. Hence why Ariados, who has a sort of ant/face-mimicing spider theme, wouldn't evolve into an electric tarantula, which is a totally unique and self-contained theme. Whereas evolution from a squirrel into a flying squirrel just makes sense, regardless of whether or not it's the case, which is unlikely at this point. I will admit "natural progression" is a weak argument, but only on the basis that it is highly subjective.
As being primarily white is one of Pachirisu's notable features as a Pikachu clone (whereas other Pikachu clones have boasted yellow as their primary coloration), as well as being a squirrel, whereas other Pikachu clones were nothing but generic rodents who don't seem to based on any particular variety of rodents, I would consider that a relatively solid connection for the archetype of Pokemon it's situated under. And more to the point, far more of a similarity than Ariados -> Dentula. I hate being grouped under that variety of idiots purely because I believed Emonga could evolve to Pachirisu for totally valid comparisons.
For the record, I'll bring up the evolution of Pikachu to Raichu again-- their tails are very different. Perhaps not to the extent Pachirisu and Emonga's tails are different, but they are still notably different. As are their ears. Which is why I don't consider details like those to be evidence against an evolution from Pachirisu to Emonga.
