Why is pikachu the only Gen 1 pokemon that stayed Japanese

Dudebusters posted...

"S" by itself doesn't exist in Japanese. It can't be Demos, the s needs to be accompanied by a vowel. The reason we know it's usually just straight up an "s" when romanized is due to the intention of the japanese word/name.

I get that when a name is clearly supposed to be an English word it would be incorrect to add the vowel.

But in that example, since the name doesn't actually mean anything, neither would be incorrect. Is that right?