It's definitely generation 9.  It is behaving just like every other "next gen" console that released early.

In 1989, the Genesis launched in North America.  It got trounced by the NES.  Then in 1991, the SNES finally launched and the two consoles were fairly competitive for several years.  Genesis was created to compete with the generation 3 NES, the Nintendo.  "Genesis does what Nintendon't."  However, history has always considered it generation 4, because it competed with the SNES for most of its life and also it succeeded the Master System which launched around the same time as the NES.

In 1995, the Playstation 1 released in North America.  It got outsold by the SNES that first year.  If the PS1 couldn't outsell the SNES for the first year, then what hope did it have against the much more powerful N64?  It had a lot of hope in fact.  It went on to dominate the market, destroying the N64, and becoming the best selling console ever (at the time).  Worldwide sales of the PS1 exceeded the previous record holder, the NES, by over 40 million consoles.  This little console that struggled out of the gate managed to far exceed everyone's expectations.

In 2005, the XBox360 launched.  It was far outsold by the PS2 for that first year.  If it couldn't stand a chance against the much weaker PS2, then how could it possibly ever compete with the more powerful PS3?  It turns out that it competed quite well.  For most of the generation the XBox360 hardware was selling ahead of the PS3, although the two did end up in a virtual tie at the end.

These are the sorts of things that happen almost every generation.  A console releases early, but it's performance against established consoles has no effect on its lifetime sales.  The established consoles are really "last gen" consoles.  Instead, the early release seems to help it's performance against the competitors that are going to come out.  The first console to market gets a head start and it seems to help.  It doesn't guarantee victory or anything, but it helps especially if the console is performing decently to begin with.  (Although, some consoles like the Saturn or Wii U do so badly that the head start really doesn't help at all.)

Switch is following this same pattern.  It released several years after the PS4, XB1 and Vita.  It's selling well, but it's still getting outsold by the PS4.  Doesn't this mean the Switch is doomed?  Nope.  It means it has a head start.  It's real competition is the PS5 and Scarlett.  People see the Switch competing against the PS4 now and assume it will compete again the PS4 forever?  That's just nonsense.  The Genesis competed against the NES for 2 years and got its ass kicked.  That doesn't change the fact that it was a generation 4 system that was really competing against the SNES.  The Switch is really competing against the PS5 and Scarlett and it already has a huge head start.

Exactly, why can't we accept that two devices from the same company can't fit on one generation?

Wii U and NSW are on the same generation as PS4 and XB1.

NES and Mega Drive/Genesis/Mark III are the same generation.

SNES was replaced by the N64 as the flagship Nintendo console during the Saturn/PS1 era.   The continuation of selling after replacement is irrelevant. PS2 was discontinued in 2013 but is not considered part of the PS3/X360/Wii (Gen 7) era.

Dreamcast is certainly a borderline case as it spent an equal amount of market time competing with Gen 5 as Gen 6 but that's only because of Sega's abrupt exit from the market.  Otherwise, it would have spent the majority of its life during Gen 6.

And for the first 4 years the SNES competed against the rest of the 4th generation.  Why are you only looking at the last 2 years?

Further, I never said 2 generations cannot be active at the same time.  You're welcome to track down a quote of me saying such.

And for the last time, it's not my logic.  It's the definition the industry has been using for decades.  Bitch to them and everybody that has worked in it now and since the beginning when they established such delineations.  I don't get why you are so angered by this.  You told me to "get over it" in the other thread as though you alone dictate how the industry should classify generations. I'm just telling you how they've done it since the start.  Take it up with them.

If we are going that route, it's Gen 7.

NES - 1
SNES - 2
N64 - 3
GC - 4
Wii - 5
Wii U - 6 (I remember people hoping they would call the console the N6 before the name was announced).
Switch - 7

Exactly, why can't we accept that two devices from the same company can't fit on one generation?

Wii U and NSW are on the same generation as PS4 and XB1.