Don't doubt that Nintendo won't do both. SCD can only do so much. At some point, the hardware becomes a bottleneck. I expect a physical hardware upgrade for the NX and NXDS once every two years each, alternating years of release indefinitely.
Most definitely. I see them releasing SCD for NX in 2019 or so in order to boost it up to around the level of PS5 and Xbox 4 which will likely release in 2019 so that NX can keep getting 3rd party ports. Eventually they'll need to release a new console though, SCD can only boost NX for so long. I don't know about hardware every 2 years though, that's pretty fast, at least for the console, handheld might need an upgrade every 2 years so it doesn't fall too far behind smartphones and tablets, but with game support lasting for 4 years.