It's true that most Nintendo systems don't last more than 7 years, but it's also worth pointing out that most Nintendo systems haven't enjoyed the success Switch has achieved. Additionally, this is their only system as opposed to have 2 at a time for the past almost 4 decades.
8-8.5 years is still possible for the Switch. (I could see the pandemic being a reason for the successor releasing in 2025 as opposed to 2024.)
I'm pretty sure the NES is famous for lasting more than 20 years. GameBoy lasted fairly long as well like 10+. Now that I think about it, Nintendo has consistently created an upgrade of the old system as their next-gen.
NES -> SNES
GameBoy -> GameBoy Advance
DS -> 3DS
Wii -> Wii U
Switch -> Switch 2???
P.S. The successor has all sold less than it's predecessor. Maybe they should stop doing that or sell them as upgrades, but at the same time N64 & Gamecube wasn't really amazing in terms of sales either. I'm assuming they would stop this trend from happening and create a complete new hardware with new concept for their next-gen console. They will still make a successor for the Switch but will be treated as an upgrade like the "New 3ds" than a next-gen console.