2021 will be Switch's 5th year on the market so sales are likely to decrease regardless of what else is on the market.

But there are things they can do to keep interest up such a price cuts, further revisions and most importantly keep releasing big games.

That's the annoying part, Switch will decline in 2021 and people will claim it's cos of PS5/XSX, when it reality it is entirely normal for a system to decline in it's 5th year.

What? I never said they weren't sluggish, I said Wii wasnt the reason, I thought I made that quite clear.

You're whole premise of "360/PS3 were down when Wii was up and 360/PS3 were up when Wii was down" makes no sense when you actually look at the trends.

2007

360-7.9m

PS3-7.9m

Wii-16.5m

2008

360-10.9m (+38%)

PS3-10.2m (+29%)

Wii-24.2m (+46%)

Why did PS3 & 360 both have ~30-40% YoY growth in Wii's peak year if Wii was the reason their sales were low?

360 & PS3 both had slower starts and peaked late but it had very little to do with Wii.

I would say they didn't even compete in SW (sure you can say their SW compete to be GOTY or anything like that, but isn't like Switch and PS4 were competing on who would receive a SW, if the game was doable on Switch it would get a release, if the game didn't need portability PS4 would get a release as well).

For me they only competed for mindshare and media coverage, but with still enough space for both to thrive (and they did, when Switch launched PS4 beat it's record year while Switch was nearly beating PS4 launch numbers at the same time).

It's normal for a console to increase in sales in it's second year after release.  PS2 sales peaked in 2002.  Both Gamecube and XBox sales were significantly up YoY in 2002 as well.  It's not the percentage increase that matters.  It's absolute sales.  

Your argument would say that PS2 was not competing against Gamecube or XBox because they all had YoY increases at the same time. (Clearly this is a nonsense argument.)  My argument says that Wii was dominating PS3 and XBox360 in 2008 just like PS2 was dominating the other two consoles in 2002.

The Wii, during its first few years, was selling just like every dominant console sells, and it's competitors were selling much lower as second and third place consoles usually do.  The difference is that after a few years the Wii sales dramatically increased.  When that happened the other two consoles got a sales boost.  That is competition.  That is how competition works.  High sales of one console correlates with lower sales in another and vice versa.

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.