R&D costs money, yes, but it costs money Nintendo is constantly spending. It makes sense to keep your R&D team fully intact between generations than try to hire a bunch of people to design your next system, when you're ready. If you're going to have those people around, they need to work on something and you have to sell that something, otherwise you're just spending cash to no benefit.
The different model approach definitely leads to more sales. When the Switch came out, there were people who thought "That's too expensive and I'd only want to play on the go", and the Lite sells to those people. There's people now that thing "Well, the Switch is cool, but seeing those resolution dips/frame rate drops turns me off", and a Pro would pull in those people. Not to mention that you also get double dippers.
Further, dropping a new version, or even having rumours, gives your product some social cache. The media will run stories, which is free publicity, which keeps your product at the top of peoples' minds.
I could see a full cellphone model working, but the systems would have to be 3/4 years apart, where only two "waves" are supported. But that assumes Nintendo wouldn't have some other hook for their next generation of consoles.