Um, Yokai Watch is Level 5 and Nintendo is the one who publish it in the West

3m shipped is basically a lock at this point. Remember, games on Nintendo platforms are rarely front-loaded, and Switch still has a relatively small install base sure to grow. Not to mention JRPG fans are core gamers, actively looking for games they have missed when they jump aboard a new platform.

And saying they don't have to engage on the Switch because they sell well on PS4 / Xbone is so absurdly wrong. For example porting KH3 to the Switch (assuming it's possible) would effectively drop development costs, and consequent risk, not to mention more revenue, sales, profit and exposure.

And it can go even better with exclusives. I dare anyone argue that an exclusive FFVI remake for the Switch would not sell multiple millions, in the range of 4-5 or even more.

If Final Fantasy and Dragon Quest don't have big legs on any Nintendo platforms, why would Octopath Traveler?

The last FF was over 10 years ago (and a shoddy spinoff to boot), and DQ 9 had some legs on the 3DS, but lot too long ones (I'm not counting DQ10 since it's an MMO)

But the main difference is that these 2 are very recognisable and known brands, so those interested buy it right away. Octopath on the other hand, being something brand new many didn't have on their lists, is something many are only buying down the line. And with all the great PR this game is getting, (total) demand keeps growing for the game

Well, there were 3 Final Fantasy games on 3DS, so it hasn't been 10 years (and each one did not have legs). There were also 10 Dragon Quest games on 3DS and Dragon Quest X on Wii U. None of those had great legs (and only three doubled their first week). 

I did this to show that JRPGs do not have legs. The only exception to this rule has been games aimed at children (like Pokemon or Yokai Watch). While games that had supply issues + great word of mouth have done much better than average legs (like Demon's Souls and Valkyria Chronicles on PS3), but even then, we're talking about only tripling or quadrupling FW sales in lifetime. The lack of legs has much more to do with genre and target market than whether it's an new IP or not.

Not sure if this has any teasing purpose from Square but the Bravely Default Twitter account twitted out their congrats to Octopath Traveler for the 1M milestone ... with this picture ...

Also,

The Bravely Default account changed their name to "Bravely ◯◯◯◯◯" from "Bravely Second" as well. The amount of "◯" lines up with Bravely Third.

— Stealth (@Stealth40k) August 6, 2018

*Although, it doesn't with the Japanese name. 

I should have precised mainline FF or at least Chrystal Chronicles. Those FF you mention are 2 Theatrythm and FF Explorers, 3 quick cashgrabs, while most of the DQ are remasters. The remasters are also the only ones who got released overseas between all of these games, and they are the leggiest. But like I said, gamers know those series very well and buy them early or not at all, depending on their preferences with the games, something that's simply not true with a new IP.

Pokémon may be aimed at kids... but honestly almost everybody I know who plays those games are adults. It's more that they have a wider appeal than being kids only, really.

You talk about just tripling or quadrupling first week sales without even knowing what the first week sales amount to WW, only how high they are in Japan. They could be 800k+ and then slowed down sharply due to lack of supplies. I really wouldn't rule out 3M+ by your own logic.