Again, you keep comparing something that is more of remake/spinoffish title expectations to something that should be franchise crowning achievement. If you actually want fair comparisons to a proper mainline title, it would be like if Pokemon 2019 doing 1.3ish million FW which would also be meh. (However the difference between Pokemon 2019 and Kingdom Hearts, is I actually seen Kingdom Hearts in motion, for all I know Pokemon 2019 could actually be an awful game). 

Also I think you are treating meh as meaning "bombing", which is the reason I started using the word "meh" over disappointing because so many people confuse the word disappointing as I think it will bomb. 

If this is about Kingdom Hearts, it should remain at that notion.  However most of your argument seems to be how I might view Pokemon Let's Go as differently (ignoring the notion that it changes a lot of what the franchise is in said comparisons). Which I also stated many times if comparing to other remakes it did poorly in Japan.  However I acknowledge it trying to do something distinctively different.  

At this point KH3 might be the spinoff sadly. It's been 14 years since a main game launched on home consoles, and as we know Japan has changed a lot since then in regards to their tastes towards home consoles. Since then, most games have launched on a handheld. I would definitely be satisfied with a 400k+ start, anything better is quite stellar.

You think the franchise was damaged by all separate experiences that it would hurt that much? That would be really bummer if that did happen, it is actually what concerns me about Ace Combat 7, it is theoretically possible to do worse than 360 Ace Combat 6, but I am also hoping that isn't the case, since it is a title I want more of. (I'm not even mentioning Assault Horizon, which had higher sales, since it was on the PS3 and 360, which is something I do not want more of.)

I will mention that the reason I think under 500,000 will be "meh" is also because I believe it will lack the legs of the first two mainline entries, as I seen countless times this generation outside of very few titles (for RPGs). Namely with the 75-80% drops on titles. 

If I recall both Kingdom hearts 1 and 2 reached 1.3 million plus, which I think if KH3 only did about half of that lifetime, it would be a bit disappointing no?

Instead of comparing pokemon we can just compare other SE JRPGs for home consoles. PS2 to PS4. My numbers might be slightly off as I don't have archives.

FFX did 2.26m in its first year. FFXV did about 1-1.1m with digital.

DQVIII did 3.5m. DQXI (PS4) did 1.4m in 4 months.

Here we see in the heyday of home consoles that the mainline title did more than double what the PS4 version did. So with some KH projections we can estimate:

KH2 did 1.1m+. KH3 (PS4) might do 400k-500k. We can of course hope for better, but the trend isn't so good.

The west saved FFXV, while DQXI had some 3DS and will have some Switch help. I think SE is banking on the XB1 version of KH3 to do alright.

Bonus:

Star Ocean PS2 did 533k in 2003, Star Ocean PS4 139k in 2016.

I do get your point, and do understand the notion of huge declines for Square. (The DQ one was really the only one that was unfair) It is so strange to see Wada's damage take effect all these years. Given the Ps3 really did not help, with how it was introduced. It is a bit sad, looking at some of these, since the majority of the ones you mentioned I felt was damaged by the HD era under Wada.  

I still treated KH to higher standard since it largely was exempt from HD era, but I do see you point. 400,000 wouldn't be too bad, it will still be disappointing for what the market was, but I guess I should lower my expectations from noticing some of those huge of declines. 

Bonus: Star Ocean PS4 deserves it for what the hell Star Ocean 4 was, never felt more bored in JRPG.