Rough break for Seattle, not only do they lose him but he goes to another team in their division. I think maybe the rumors that he wanted a team who hadn't had Japanese players before ended up being true.
You know what, in retrospect, this makes a lot of sense. Ichiro is probably the best player to come to the US from Japan, he's still beloved in Seattle and so his shadow was going to hang over Ohtani, no matter what he did. Especially considering that what Ichiro did was so unprecedented, not just from a Japanese player, but really from any MLB player. He's a first-ballot Hall of Famer that was an immediate cultural phenomenon.
Could Ohtani do that? Maybe. Does he want to deal with that extra pressure of being Ichiro? I wouldn't. Why start in a place where there is already a huge shadow when you can play in the sun of Anaheim. This same thing will happen to the guy who takes Brady's place as Patriots quarterback one day.
The worst job in sports is replacing (and I know that Ohtani isn't a direct replacement for Ichiro, but he'd be the next big Japanese import) a legend. If you don't have to do it, don't.
Edit: Plus he gets to play every day with Mike Trout and not have the burden of carrying the franchise on his back. Ohtani is a smart dude.