The thing about Ash-Greninja's transfer potential is that it's not an inherently unique Pokemon. In HGSS they went out of their way to explain that they wouldn't let you trade it because it was affected by time travel with Celebi. Ash-Greninja is, lore-wise, a regular Greninja that happens to have a mental bond with the trainer to fuel the transformation. That said, the fact that it is acquired by transfer from the demo alone indicates that it can form this bond with different trainers, so it should be capable of making this link and becoming Ash-Greninja if transferred to a different generation.
The bank evidence is also a strong point as well, but I think a few details should be mentioned.
- Cosplay Pikachu was in the "remakes" for the gen, so there was unlikely to be a main title after wards for it to transfer forward to, while Ash Greninja came out in the first (possibly only?) main games of Gen 7, so there might not be a reason to restrict its transfer if they decide to arbitrarily make it exclusive.
- Ash-Greninja is very heavily associated with a different region than the one in which it made its game debut. Spike-Ear Pichu did debut in a Sinnoh work (Arceus movie), but it was honestly just kinda there and didn't tie into anything significant. Ash-Greninja's bond is parallel to AZ-Floette, and I recall it being mentioned/speculated it bore a resemblance to Infinite Energy like the Ultimate Weapon in Kalos. There's no lore justification for tying it strictly to Alola when Kalos already passed.
- Ash-Greninja is a promotion marketed as "look how strong and awesome this is" rather than a marketing gimmick and "trophy" mon. The Pikachu promotions are things mostly thrown out because GF insists on making sure you remember their mascot is relevant, and while Greninja is popular, he's not the face of the brand that GF can throw out a gimmick for so casually.
- Access to Ash-Greninja is theoretically unrestricted. The Pichu promotion came from getting a different event that has since ended, then bringing said event Pichu to the Ilex Forest to get Spiky Eared Pichu. It was promotional and not likely to be missed. Ash-Greninja on the other hand came from the demo, and thus should be available as long as the 3DS eShop is active (which means it will survive as long if not longer than Bank), so even late arrivals could come for it. It's also not some massive mechanic they ahve to explain like Mega Stones or Z-moves, it's one unique Greninja they could just toss in the way they do things like the Relic Song/Secret Sword tutors, the Reveal Glass, etc. Given the anime it could even tie into how they bring back the Zygarde forms.
The bank evidence is also a strong point as well, but I think a few details should be mentioned.
- Cosplay Pikachu was in the "remakes" for the gen, so there was unlikely to be a main title after wards for it to transfer forward to, while Ash Greninja came out in the first (possibly only?) main games of Gen 7, so there might not be a reason to restrict its transfer if they decide to arbitrarily make it exclusive.
- Ash-Greninja is very heavily associated with a different region than the one in which it made its game debut. Spike-Ear Pichu did debut in a Sinnoh work (Arceus movie), but it was honestly just kinda there and didn't tie into anything significant. Ash-Greninja's bond is parallel to AZ-Floette, and I recall it being mentioned/speculated it bore a resemblance to Infinite Energy like the Ultimate Weapon in Kalos. There's no lore justification for tying it strictly to Alola when Kalos already passed.
- Ash-Greninja is a promotion marketed as "look how strong and awesome this is" rather than a marketing gimmick and "trophy" mon. The Pikachu promotions are things mostly thrown out because GF insists on making sure you remember their mascot is relevant, and while Greninja is popular, he's not the face of the brand that GF can throw out a gimmick for so casually.
- Access to Ash-Greninja is theoretically unrestricted. The Pichu promotion came from getting a different event that has since ended, then bringing said event Pichu to the Ilex Forest to get Spiky Eared Pichu. It was promotional and not likely to be missed. Ash-Greninja on the other hand came from the demo, and thus should be available as long as the 3DS eShop is active (which means it will survive as long if not longer than Bank), so even late arrivals could come for it. It's also not some massive mechanic they ahve to explain like Mega Stones or Z-moves, it's one unique Greninja they could just toss in the way they do things like the Relic Song/Secret Sword tutors, the Reveal Glass, etc. Given the anime it could even tie into how they bring back the Zygarde forms.