Splatoon has really ****ed up peoples' perception of both what a successful new IP is, and what the barometer for entry is, huh? Splatoon has sold like 15 million units with only two titles - it's more successful than most series on the roster.
I mean, ARMS would've been included on the roster, and that one game sold less than half of what one Splatoon title sold, which itself was still over double that of what Xenoblade sold when Shulk got in. (yada yada sales don't equal inclusion, which is true, but it's a rough metric for audience size, which matters)
And by Smash 6, two games after the likes of Xenoblade got in, the bar is going to be that much lower. No IP needs to perform even near what Splatoon did. Splatoon is an aberration, the success of which even caught Nintendo by surprise.
If Astral Chain can get a sequel, it'll probably get in based on the virtue of simply being a current IP with multiple titles.