The release of one system from another brand has never had any impact on sales of a console from another brand, either from the same generation (for offset releases) or from a previous generation.

The evidence for this claim is strongest for cross-generation cases. Despite Sega claiming to do "What Nintendon't," the release of the Genesis had no effect on NES sales. The release of the PS1 had no effect on SNES sales. The release of the Dreamcast had no effect on PS1 sales. The release of the 360 had no effect on PS2 sales.

Even within generations, there' strong evidence that a console's sales curve does not experience a sudden decline upon the release of another system later on. The available data we have on 16-bit consoles shows that the Genesis experienced significant growth in the U.S. even after the release of the SNES. The PS1 experienced sales growth despite the release of the N64. The release of the PS2 had no effect on Dreamcast sales, which were actually up for most of 2001. The launch of the GameCube and Xbox had no impact on PS2 sales, which were very strong in the 2001 holidays and were up a good bit in Q1 2002. And the Switch had no effect on PS4 or XBO sales.

With the 360 there is weak evidence that the release of the PS3 may have hurt it some as it was down for the April-July period of 2007, but that is the only instance I can find of a console experiencing any kind of significant drop in sales from the release of a same-gen competitor within one year of said competitor's launch, and given that it's the lone example it could just as easily be that the decline could have been due to other factors. In fact, it is worth pointing out that the 360 had supply issues for its first few months that depressed Q1 2006 sales, but the issues were resolved by April that year, which produced an unusually strong Q2 (the second quarter is usually the weakest of the year). Oh, and the 360's explosive growth in 2010 was due to the release of the 360 S, not the decline of the Wii. The delayed peak of it and the PS3 can be explained by factors specific to those two systems, without needing to invoke the Wii.

There is absolutely no reason to expect that the PS5 and XSX will have any effect on Switch sales. The console market is not a zero-sum game, especially when it comes to Nintendo vs. PS & Xbox. This is not to say that competition has zero effect on sales. It does, at least when two consoles are offering very similar or nearly identical experiences. But there is no evidence that the release of a system from one brand has an immediate and negative impact on the sales of a system that has already been on the market for a good while if it is from another brand.

Normally you seem to know what you are talking about, but these statements I bolded make it seem like you don't believe in competition.  Don't you think any of the consoles are competing with one another?  Why did the SNES sell less than the NES.  Could it be that they bought a Genesis/Megadrive instead?  Why did the PS3 sell less than the PS2?  Is it just a coincidence that the XBox brand sold better in Generation 7 while the Playstation brand sold worse?

I mean, think about what you are actually saying.  These consoles are actually competing.  If one company makes a misstep, that doesn't mean most people just stop playing video games.  They buy a competing console instead.

Are you seriously denying that video games of the same generation affect each other's sales? The ps5, xsx and switch 2 will obviously affect each other just like wii u vs ps4 vs xb1, wii vs ps3 vs x360, dc vs ps2 vs gc vs xb...

Yeah, like DS did affect Xbox 360 sales, GBA did affect PS2 sales or 3DS did affect PS4/Xbox 1 sales. Also, fans of Killzone/God of War/Halo/Gears of War/Fifa/CoD/GTA/Battlefield and fans of Mario/Zelda/Animal Crossing/Fire Emblem are the same. 

Not really, the effect on PS3 in Japan was a change in the market landscape.

How so?

PS2 (23m) + GC (4m) + XB (0.5m) = 27.5m
PS3 (10.5m) + Wii (12.8m) + 360 (1.7m) = 25m

Yeah, like DS did affect Xbox 360 sales, GBA did affect PS2 sales or 3DS did affect PS4/Xbox 1 sales. Also, fans of Killzone/God of War/Halo/Gears of War/Fifa/CoD/GTA/Battlefield and fans of Mario/Zelda/Animal Crossing/Fire Emblem are the same.