We assume it because we've never been given anything to tell us otherwise. It's not that it couldn't happen but in current canon he is gone.
The fact he's never brought up by characters adds more fuel to the fire that he no longer exists.
It's true that we never got anything regarding Riku Replica
himself to tell us otherwise, series-(or in-universe-)-wise we have more than enough cases which show us that fading into/being swallowed by darkness does not necessarily mean "That's it!"
In the current canon though, Eraqus, Vanitas, Roxas, Xion and Naminé are
gone too.
This is also one of the more iffy things in the series because of the different interpretations(and statements in the games) of what "not existing" actually means.
For all intents and purposes, the characters I mentioned above "no longer exist" as well in the terminology of the series
right now, just like the Riku Replica, as they do not have a physical form anymore. Nomura even went so far to say that everyone who called out to Sora in Blank Points has "already disappeared" from the world and that even Aqua (and Ansem the Wise) would have a "Birth by Sleep".
Well for all intents and purposes, she was "dead" (using that term loosely). Before KH2, that was canon.
Possibility isn't fact, though. We can theorize all day about the possibility of his return but we have no solid evidence of it, which is where I'm coming from.
It's sad. :C I think that's why the fandom feels such a strange connection to him.
Yeah, I also think we're also having a slight mismatch in semantics here. Maleficent was
gone until KH 2, but in Chain of Memories Vexen already alluded that Maleficent, while gone, would not necessarily have to remain gone.
I was not doubting anything in regard to Riku Replica being
gone, but what actually makes the fandom think it's a sure-fire case of
gone for good.
Correct, but saying that he
cannot return and is surely and utterly gone for good isn't fact either,
both are just possibilities and in the case of the latter one becoming true, I am certainly interested in the explanation as to
why exactly Repliku is completely gone for good.
Thinking the whole thing further, that
may also hold interesting information in regards as to how to get rid of Xehanort for good.
That's understandable, the only thing I do not like is however when Repliku is used as an entry point for Xion hate/bashing, that's not fair to both of the characters.
I also just thought about it, the Replica program was initially thought up by Xemnas, who is a Xehanort incarnation, and both Repliku and Xion were just used as tools in a scheme.
In BBS, Master Xehanort uses Ven and Terra as tools in his schemes, in KH 1 it's Ansem with both Riku and Maleficent, CoM has Marluxia hijacking a plot that was also originally envisioned by Xemnas/Xehanort, using Sora, Naminé and Repliku.
In Days, Roxas and Xion were used, in KH 2 and DDD Sora again.
Sora's "treating hearts like bottles on a shelf"-statements in DDD really fit through the whole series on what Xehanort has been doing all the time.
