Well, I honestly was expecting Ash to use Lugia in his final showdown with Leon, but I guess Solgaleo will have to do.
I'm not too surprised seeing how Ash did help raise it as a nebby, and befriended it after it evolved. After all, Ash is without an Alolan Pokemon on his current team, and it seems fitting that an Alolan Legendary, such as Solgaleo, properly represents a pokemon of the region he's champion of, the same with Leon, getting Eternatus.
I guess this means Solgaleo will officially be Ash's first legendary pokemon. Whether he keeps it or lets it roam is unanswerable.
However, Solagelo is not as majestic as suicune, but at least its more powerful. Its a shame Goh will be allowed to also get Regigigas to equal the playing field.