There wasn't foreshadowing on Marshal's part. All he said was:
Marshal: My master told me about this, so I came to see. Do you understand? Even if you lose in battle... As long as you surpass your past self, you've still achieved a victory over yourself. So for that purpose, through battle, we see opponents! We see Pokémon! And we see ourselves!
Cheren: ...I suppose that's a line you're passing down from Alder?
Marshal: ...Well, yeah. What I want is to fight a Trainer who sees themselves that way... In associating with such a Trainer, I know I and my Pokémon would get even stronger...
For Marshal, surpassing oneself is victory enough. He doesn't need a title. Just like how Banjirou doesn't need a title. There's got to be a lot of irony for Marshal to be passing on those words of Alder, and then wanting to be Champion when Alder himself gave up the role of being the active Champion during the BW-era to travel the region, only returning to that seat in an attempt to foil Team Plasma's plan.
Also, if Alder wasn't beaten, then it would make sense that the next strongest trainer in the League, one of the Elite 4, to replace him until a Champion is found and another trainer get promoted. Saying
Iris didn't beat Alder just makes it seem like they plucked a random trainer who had not proven her qualifications to beat the league. Yes, Iris had potential, but unless she showed it by beating the League, I doubt the League would have chosen her over Drayden.
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