Actually, someone did already.
When you think about it, he just makes the most sense.
Interesting. Never knew this. In my earlier comment I was technically refering to the Gen 1 -8 games, but I'll take Professor Oak owning a Mew.
To be honest, I would not be surprise me if the anime does reveal Professor Oak to be a previous owner of a Mew in which he releases after working to study it. After all, Professor Oak was revealed to own a Dragonite, and its hinted even in the Kanto Gen games that he use to be an elite trainer whom one of the Elite Four, Agatha, knew of.
It wouldn't again be a surprise at all if the anime reveals that Professor Oak did become the first Pokemon Master or one of a few trainers that did, and that he even had also caught a Mew sometime ago when he was a trainer but ended up releasing it. I also wouldn't be surprise if the Mew Goh saw on Professor Oaks research ranch was actually a Mew Oak use to own.
With that said, if Goh does catch Mew, it will not be too big of a shock for me, seeing as he's already caught many strong pokemon.
Unlike Ash's accomplishment of defeating Leon, I'm very curious how most fans would react to Goh's accomplished.
I think we've come to the point in the anime that capturing legendaries will become quite a common feat, just like how several characters now are shown to possess Dragonites, which was at a time considered legendary.