The big difference between the Switches 2021 and the Wii's 2011 and the Gamecube's 2005, is that we have no information on new hardware coming from Nintendo or even a rumored codename at this point. With the Wii, we knew that Iwata was working on a new console in 2004 and there were rumors even before this because of how poorly the Gamecube was performing. With the Wii, Wii U rumors were public in 2010 according to Wikipedia. With the Wii U, we heard from Miyamoto in 2014 that Nintendo was working on new hardware and the NX was formally announced by Iwata in the first half of 2015. With the N64, we knew about project dolphin around E3 of 1999, 2.5 years before the Gamecube was released.
With the Switch we have no rumors of any new hardware from Nintendo and we are already into 2021. If anything, the lockdown from the last year has set back any R&D that Nintendo was doing on the Switch's successor and based on past trends it takes at least 2-3 years after we receive a codename from Nintendo before a console is released. I really think that the Switch has at least 3 years left in its life and Nintendo is much more likely to release pro models or faster variants of the Switch rather than going with totally new hardware, in which case the Switch could have an entire second life ahead of it in terms of sales and software. I do not see 2021 as being anything close to a twilight year for the Switch.