I know Xenoblade has never been really popular in the mainstream. Spiritual Xeno games related to Xenoblade were more like sleeper hits
Xenogears (PS1) is at 1.7 million, a decent number , while Xenosaga Episode I (PS2) sold 1.6 million

Both Xenosaga II and III were flops, while Xenoblade Chronicles 1 (Wii) and Xenoblade Chronicles X (Wii U) were really limited in sales by external problems (With the first: Late localization, lack of advertising, late in Wii life cycle and limited copies;  X was born in a dead console with late localization) (both sold 0.9 million worldwide). Although this may be unsettling for the future of the franchise, Xenoblade Chronicles 2 is the decisive step, i think

Mi prediction: 450k FW, 1.5 million LT 

My reasons: Although trailers are kinda weak and i haven't seen a lot of advertising other than E3 and directs, localization will be inmediate , and it will launch in the holiday season. This means Nintendo is giving it the oportunity to shine.  Switch is being a hit, unlike Wii U and Wii at the time previous entries were released. And this prediction is considering Xenoblade Chronicles 2 is a solid game (hypothetical).

Lack of solid JRPGs on a smash platform everyone's eager to buy games for, during the holiday season. Seems like a recipe for success to me. Slow burner to 2m LT.

Wow your prediction is the same as mine
FW:450k
FY: 800k
LT: 1.5M

Hopefully it will be the Fire Emblem Awakening of its saga

Hopefully 2m LT but most probably 1.2-1.5m LT

Very tough to call. So far, they have been pretty gung ho about it, but we have to wait and see if they close the lead up to release out with a big marketing push or if they go gun shy at the last second like they did with X. Hopefully they do promote it, I think both this game and the series as a whole has more widespread appeal potential than has been exploited thus far. Current trends rught now is every major retail exclusive is getting a push so that's a good sign.

And of course, the Switch is on fire vs the Wii U which was dead and the Wii which was in its twilight.

Xenoblade has had some good bursts of activity on Amazon, Gamestop, and other retailers so that's a good sign at least.

Also, the game launches more than one full month after Odyssey and in one of the highest spending periods of the year. That gives it some breathing room and a lot of possible sales to he had. It also means Xenoblade 2 will be the big exclusive of November-December which could work out very nicely for it given how many new Switch owners there will be in that time.

Overall, I'm optimistic. I think this will be the one to break 1 million, could go well beyond but we'll see. 1.5 seems reasonable.

Hope lifetime sales go over 2m.

Yeah, at the start, X's advertising was really good
At its launch it was absolute silence

1.8 Million LT.

It should do better than its two predecessors, based largely on the factors you mentioned, but honestly, the marketing for Xenoblade 2 has been really, really poor.

Their latest Direct in fact turned my roommate off from the game entirely and made me skeptical about buying it. Generally, trailers and presentations exist to get you hyped, but Nintendo's approach with this game has had the opposite effect, at least on me and the people I know who were interested in it.