I agree. The people claiming Greninja will be released are people jumping to conclusions or want the worse case scenario to happen.
I don't think it's a big stretch to believe Greninja might be released considering Ash is looking sad and is actually giving Greninja both a handshake and a hug while the sun is setting. While it's not 100% confirmed, it's still reasonable to speculate that it's leaving due to how the scene looks. Especially since I personally can't think of any reason why Ash would give a handshake before hugging unless it was a farewell.
Now usually I would think that being released is like the kiss of death but after Goodra came back, maybe it's not so impossible for Greninja to reunite with Ash if it does leave. Granted, Goodra has a person that can send it back to Ash if need be and there are things like pokeball transporters that can be available. However, with Greninja being in his Ash form, I'd guess that if it were to leave, it'd still retain that connection it has with Ash and maybe if Ash would ever need Greninja, Greninja could sense that and use the power of friendship to reunite with him somehow. Honestly, the writers can come up with anything really if they wanted to bring it back.
Not like it matters too much anyway. Honestly, even the pokemon at Oak's don't get many appearances and seeing Sun and Moon, Ash doesn't seem like he'll call on any older pokemon anytime soon. Ash's access to Greninja for any future battles would really only matter if Ash finally and officially faces a Champion. Which let's be honest, that's not happening for a really long time either.
While it is hard to think of Greninja potentially leaving, at the same time, I can see it happen given the circumstances. Months ago, this would be something I'd absolutely be disappointed by but I guess since it looks like Greninja is in Ash form, just knowing that it'll always have a part of Ash with him makes a potential release okay for me.
