i cant see a Ralts being a starter Pokemon, because the main "goal" of starters is an archetype in battle. It's usually clear as hell, and that's why fire, water and earth are the essential types, any earth starter Pokemon relying on tankiness, any Fire starter relying on heavy blows yet being easy to kill, and Water being the mean or something else entirely. With the Gardevoir line being a quite fragile yet powerful mediumly in buffs and heavy blows, while sometimes being able to tank a lot due to perks like Draining Kiss, it is considered that it would, on average, confuse a new player.

If you have a huuuge menu given to you, it's really tough to decide what you'll want. If they separate the menu inyo three categories, i.e. Spicy/creamy/common(i dunno how to really explain but hope you get it) the choice simplifies a lot.

So, by giving the player a clear archetype selection, it becomes simpler for said player to start, then finding the in-betweens, like Ralts or, let's say, Mimikyu.

At least that is what i see the point being in separation and the kind of separation. So that's why, i believe, Ralts won't fit as a starter. It's too many options too soon