I really hope Xeno 3 ends up outselling no effort products like Switch Sports and Mario Strikers, not holding my breath though.
Xenoblade might get pretty close to Mario Strikers, since it seems like it had the strongest debut in the series, at least in some regions. Strikers might have stronger legs (ugh), but word of mouth is pretty tepid, so maybe not. No hope of it coming anywhere near Switch Sports though, of course.
What's your reasoning, here? Even if Nintendo misses their 21 million goal by a bit, the Switch should still be at 125 million+ by the end of the fiscal year. That would leave it 30 million shy of the PS2 and DS, which means it would have to average 15 million for the following two fiscal years to get there. The Switch hasn't had a (full) fiscal year under 15 million yet, so you would need a pretty dramatic drop to take it out of contention. Now, that could definitely happen, especially if Nintendo announces a successor soon, but we currently have no idea if that's going to happen, since they could just as easily announce another revision that's still part of the Switch family, which would *increase* its chances of surpassing the DS/PS2 rather than decreasing them. And, also, there's always the chance that the Switch successor will be a very similar device with backwards compatibility and that Nintendo will continue to release games for the OG Switch, slash the price, and market it as an entry level device for a few more years. We really do not have the information we need to say anything about sales beyond the end of this year.