Losing in the first battle is probably to try and make you hate your rival. Remember, this is a game meant to be 'the pokemon that fans want' - aka, the Pokemon game fans have been asking for. Which means it's geared towards all the squeaky wheels complaints. Don't like friendly rivals? Here's one that artificially beats you in the first battle! Don't like critical hits? Bam, all luck removed from battle.
I mean, I've seen the first seventeen minutes of gameplay, and they make the guy insufferable.
Locking you into a loss wouldn't make you hate the character, rather, it runs the risk of hating the game itself. Take Blue/Gary for example, he was still hated because of his attitude, his egotism, telling you to smell him later... The fact that he was always a step ahead of you. Then there's the yellow version, where he outright kicks you out of the way and takes the pokemon you were to get.
And you could still beat him in your first battle, and it wasn't as luck based as people make it out to be. Your starter either had Growl or Tail Whip, something that could objectively give you an edge in the battle by lowering your opponent's stats.
As for the luck thing, I think that could be an issue down the line - yes, luck can be bullshit at times, but by removing any and all chance in the game, it can streamline the game to an Absurd degree. It's happened in games like Yugioh and Overwatch, where they made the games extremely cookie cutter that it only came down to you having certain teams that resulted in a rock paper scissors match up (Yugioh especially). I'm not saying that it's going to be that way, but that it has the chance of extremely streamlined to the point where there are just an objective small amount of Temtem to actually use.
Also, for everyone complaining about the lack of player choice in pokemon games and such, I find it to be really dumb to be locked into an automatic loss. At the very least, they should give you SOMETHING that could give you an edge.
Given the trajectory of the franchise of who they pander to these days, it'll be the former option. I guess it doesn't matter regardless, all the wild mons will probably still be lv. 60-65, plus Chansey/Blissey will be there for easy conventional grinding, so even if by some chance the NPCs are higher leveled it'd take a minimum amount of effort to get one's story team up to speed.
I think it all depends on WHEN you can access the areas. If you can access it early game, then yeah, that'll be an issue in that regard. If it's when you at least are the champion, then I don't see much of an issue. Then again, this is under the assumption of when you can access this content, which a lot of people are considering post game content.