Prior to the Switch's actual reveal then newly-appointed president Tatsumi Kimishima vaguely described the code-named NX as a "long-term platform". Few months and a final unveiling later, he said that he sees the Switch as a platform for all sorts of accessories. That leads me to my theory, could Nintendo be planning to keep the Switch around for a long time? If you look closely at the design of the Switch, everything about it from it's form factor, to controllers, to branding, to the choice of SoC makes it feel like the platform was deliberately designed to be milked for as long as possible.
First, let's take a look at that SoC, it's a slightly under-clocked Tegra X1 provided by Nvidia. It's been said to that Nintendo's partnership with Nvidia was to last around 10 years. Naturally, Nintendo will want to evolve the Switch as time goes on, and release newer more powerful Switch models when they need to. With the Switch, making newer more powerful systems is actually super easy. Just swap out the X1 for the newest Tegra chip then go from there. Knowing Nintendo's habbit for hardware revisions, the ease of Nvidia's off-the-shelf Tegra hardware allows the Switch to remain competitive, while maintaining compatibility and consumer relations.
Second, the modular nature of the Switch itself. Both the Switch console and Joy-Con controllers have rails on their sides, with connectors embedded at the end. This is how the Joy-Con are able to connect to the console for handheld mode. But these ports also allow Nintendo to introduce all sorts of add-ons, peripherals, do-dads and what not. Now sure, I wouldn't want them to go overboard with way too many useless and gimmicky hardware attachments, but thanks to Switch's modular nature, it makes it easily adaptable to anything Nintendo wants to do with it.
Finally, the branding of the Switch itself. We all know the sound, the satisfying click of the Joy-Con in the Switch logo that plays right before any form of promotional material for it. It's simple, universal, and timeless. I can't help but feel that was deliberate. If Nintendo is planning to roll with the Switch for as long as they can, they would need to go with branding that lasts an equally long time. Something that everyone will instantly recognize and be drawn to, regardless of how long it lasts. So a simple click, sells the concept of the Switch in a mere second, and it never really gets old. It's the kind of branding that doesn't really have an expiration date, and can transcend decades. When you hear that click, you instantly think of Nintendo Switch. Even the name, Switch, has such a broad meaning and lasting appeal. Because you're not just Switching from the TV to the tablet. You also Switching how the controllers are held, which controller you use, which attachments you have on them, what games you will play, where you will play them, who you'll play them with.
So yeah, I don't think Nintendo is retiring the Switch as a platform anytime soon. I can see them rolling with it for a good 10 years at least. It really feels like Nintendo wants this thing to have really long legs. This actually wouldn't be the first Nintendo system with an unusually long life span. The Game Boy lasted a near decade on the commercial market without anything even resembling a successor in sight until 1998, and even then it was more of a marginal stop-gap upgrade than a true next generation.