I'm down for a legendary with pre-evos, but why two of them? I feel like, if they're going to break their long-standing tradition of not having legendaries evolve, it will probably be a big important climactic moment when it does so. The player will not have control over when it happens. Meaning, for example, they wouldn't make it evolve by level, because if they did, someone could force the climax to happen early by grinding a lot. so if it evolves at all, it will be triggered by a plot event, but it seems weird to me that they'd force it to happen twice. I can imagine a moment in a plot where one weak protolegendary has to evolve into a big strong legendary to surpass some challenge. It's extra-climactic, because it's never happened in the history of Pokemon. But, I can't really imagine a moment where one weak protolegendary has to evolve into another slightly-less-weak protolegendary to advance the story. And then, later, the legendary evolves again, and they've ruined the extra punch the final evolution will have by already demonstrating a legendary evolution.
Evolving legendaries also causes logistical problems. Like I mentioned, they're obviously not going to evolve by any means that the player has control over, or else they can't actually be related to the plot. It has to evolve at a specific plot event. But, they're in the Pokedex, meaning you can actually own them. If you have starmarble on your team and actively worked to train it, it's going to be higher-level than if you kept it in the box the whole time. But, Solgaleo and Lunala are going to have different movesets. If you leveled your starmarble up, you've passed over levels where Solgaleo and Lunala could have learned new moves. When you get to the plot event where it's time for starmarble to evolve into whichever legendary is in your game, are you worse off in the long run than the person who kept their starmarble in the box as much as possible and never let it see combat? On the other hand, is the player who kept their starmarble in the box undercutting the climactic punch of whatever confrontation comes after the evolution occurs, because their Solgaleo/Lunala is way lower level than the rest of their team? Or, there's the possibility that their levels are set to a specific number when they evolve, but then doesn't that undercut and invalidate all the work the player who trained their starmarble put into raising it?
It seems more likely to me that they'd be just different Pokemon which are thematically related to one another, but not actually evolutionary relatives. Like, the poot is the legendary responsible for the power of the moon in Pokemon Sun, and the starmarble is the legendary responsible for the power of the sun in Pokemon Moon. They're important cosmically, but Solgaleo and Lunala overshadow their powers in their respective games. Hence, my theory is that Pokemon Sun players get the first one in both the [poot-starmarble] and [Solgaleo-Lunala] pairs, and Moon players get the second one in each pair. Hence, the demo, which is Sun version, shows the poot, but not the starmarble. Poot and Lunala are still related by virtue of overseeing the same cosmic force, just like starmarble and Solgaleo, but it explains why the order is the way it is in the Pokedex.
