This argument isn't exactly valid, Ash hasn't had 3 starters since Jhoto either. Also I'm pretty sure GalladeRockes meant not fully evolved. (I made that mistake earlier as well
)
In Sinnoh nobody had a Chimchar except Paul which Ash later captured, he was the only way to show Infernape. Paul also had Torterra, but how often did he use it? And then all these other mains had a member of the Piplup line, it wasn't too hard to showcase it then
In Hoenn, Ash had Treecko, May had Torchic and Brock had Mudkip. Treecko & Torchic evolved once, and then didn't evolve until a filler. Blaziken was shown before against Charizard in Jhoto. Harrison(Or was it Tyson?) had Sceptile to show it off as well.
Now currently: Bianca has Emboar, Trip has Serperior & Burgendy has Dewott. What might happen will be a repeat with Barry and Kenny, Kenny had Prinplup which later evolved into Empoleon while Barry always had an Empoleon from his debut. Burgendy has Dewott for now, the new rival could have a Samurott, and Burgendys could evolve, which leaves Oshawott to be like Piplup sadly...
Comparing to past regions won't really work with this region, Ash actually rotates members, he has 3 starters, and there isn't a Brock!
We really need confirmation of this rival :/