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The final boss was basically a free cutscene unless you were trying to lose.
But I loved it. The Eternatus fight was the first time I felt like I was fighting a real final boss in a Pokémon game. Sure the Champion fights have a level of intensity, but they didn't feel like I was facing a world ending threat.
The two best examples of this not working are Mewtwo and Kyurem, both designed to be the BIG final Pokémon (Or boss), but always felt underwhelming. Especially if you have a Masterball against Mewtwo.
Another good example of this fight done poorly, is from Zelda actually. The final encounter in BotW felt like such a massive letdown in so many ways I can't even word it. He just walks in circles and occasionally shoots a laser, that is rarely aimed at you. It's basically an "auto-scroller", same as Eternatus, but the feeling is worlds apart.
SwSh had issues, is clearly had plot changes for the worse, but to this day, I struggle to find a game in the series that left such a lasting impact on me. And that final fight is a big reason for it.