I disagree. If that were the case, they could've just released Eternatus after communicating with it (and probably show Eternatus powering up all the Dynamax spots across Galar) or something like that. The fact that they explicitly have Goh handover Eternatus and asks Leon to look after it; in addition to Leon sending Eternatus out through a Pokeball; suggests that there is a very good chance he could bring it into the battle with Ash.
I mean they could’ve had different plans back then, but decided to wrap things up like this once they realized they’re running out of time? Time will tell really.
Well Eternatus is technically a new team member right? I disagree about it undermining Charizard's position. Base Charizard was shown to be more than capable of holding its own and even getting the better of Base Eternatus. G-Max Charizard would have likely even beat Eternatus.
I mean, we can obviously assume Leon isn't going to pull out Eternamax Eternatus. Outside of that; it just fits as a high tier pokemon on Leon's team.
Leon using Eternatus would be a lose lose situation. If he wins 6-5, then he’d have won by using a legendary Pokémon that was gifted to him the day before the finals, which would basically be another Tobias situation in that Ash was robbed of a victory due to a legendary DEM.
If he loses 6-5 despite using Eternatus, people would rightly question his strength. Ash barely won against Cynthia after all, so him winning against a Rillaboom that’s capable of sweeping Diantha, Eternatus AND Leonzard with the same team would be crazy even for JN standards.
Unlike in SM, where Pikachu had a rivalry with Tapu Koko (and where Ash/Pikachu really had no other rivals on their level), there is no reason why Eternatus should be used.
…not to mention that Leon’s biggest regret in the games and the anime was losing to Eternatus. Him using a gifted Eternatus just doesn’t suit his character.