We need to get the idea of what is balanced and what isn't in in-game tier listings. Obviously, there is no standard as everything popular seems to be avoided. Here's the issue at hand.
We need to figure out what is going to be considered powerful and what isn't going to be considered powerful out the gate. Let's use episodes as examples. Prior to episode 10, Starly was made available. This was changed later on as Starly is no longer available. Now, if we want to introduce Starly again, it would be too late. No band or life orb means Staraptor's power is mitigated by a lot, and Starly's raw power is actually fairly average out the gate compared to other arguably more useful choices such as Conkeldurr, Magnezone, Scrafty, Amoonguss, Arcanine, and Excadrill. Even if we wanted to train Starly at a low level, it would introcude back grinding which arguably kills gameplay quality.
Popularity doesn't always mean amazing. Talonflame, Ferrothorn, Slowbro, and even Scizor has been pushed back way to long despite all of them being sub-par options, at least now in episode 12. Despite what you see in Competitive battles, all of these pokemon in-game aren't godly. Scizor and Talonflame lack items and their damage output combined with their paper-thin defenses are severely lacking even after a Swords Dance. Slowbro and Ferrothorn lack some of their favorite moves due to breeding and TM purposes, and worse, both of their initial movepools are bordering niche.
There are many more examples I can think of but the point is this. Without a pokemon's favorite item, they become borderline stale compared to what exists already. I respect how balanced you want to keep Reborn Ame, but if you goal is to give a riveting and challenging league, popularity cannot obscure balance. Even if a popular pokemon does get released in later episodes, you now have people grinding to better their team; and that is never fun.