"Willing to Let go" lmao. Well I mean Suicune was caught by the hunters with a net.. nevertheless, it's a stupid argument. How is it selfless? Suicune could have broken out of the Pokeball if it wanted to. Pokemon are capable of coming out of their Pokeball; also disobeying/abandoning their trainers.
Doesn't matter if Suicune could break free or run away. At the time, that wasn't what Goh was thinking about. He wasn't weighing his options or scheming to find a way to keep it. Goh offered to release Suicune because he thought that was what it wanted, and he was perfectly fine with it eventually leaving him.
Goh deserved to catch Eternatus? The only trainer in the Franchise who deserves to catch a Special Legendary like Eternatus at this stage, is Leon (Considering he was after all the strongest trainer in the world at the time and had the strongest Trainer owned Pokemon). They could've simply had Eternatus calm down and fly away instead of Goh capturing reason.
Already mentioned, but Goh caught it after helping the others investigate Darkest Day, dealing with Rose's henchmen, defeating Oleana and joining up with Ash and the mutts. It was also a reference to the games.
And after all that, he just got a Dex entry. He never used the legend for himself, and immediately had it in the care of others. Is it so bad that he got a legendary after working to help stop it, never got to use it, and chose to give it away to someone who thought it would help the Pokemon? At this point, you're complaining about the legend's status, when the more important thing is how that Pokemon was used.
As you saying there is enough reason to Equate Goh to Gary now? Plus, Legendary Pokemon still hold a special place in the franchise. People are excited to see them being used in actual battles. It is a wasted opportunity on Goh.
Gary and Goh aren't equals, but they worked together to defeat the troublesome traineds and overcome the golems. Goh isn't as skilled as Gary, but he earned his capture. And this came after three failed attempts at battling the legendary birds.
And people would also be excited to see them captured. Regis also aren't on the same level as other legends.