That is my theory, but more for power than energy consumption. Nintendo needs to pack the handheld NX with as much horsepower as they can. By releasing the homeconsole first, they gain time to tweak the handheld's specs more and to avoid creating an even more demanding home console.
In theory yeah, they could go 28nm for the console processor for now for example, but maybe they are waiting for matured 14nm (or gulp, even 10nm) for a portable.
Maybe NX itself is some kind of non-traditional "console" to begin with that is portable in itself.
The 3DS could then stick around as a cheap entry level portable for a few years, it doesn't need many new games since the eShop could be expanded quickly so that it has a playable catalog of thousands of DS and 3DS games.
It's really weird that we've heard basically nothing on the 3DS successor, even the DS which was selling wwaaaaaaaaaaaaaaaaaaaaaaaaay better than the 3DS had it's successor already announced by about this point in its lifecycle, yet we've heard nothing on a 3DS successor. No word of dev kits, nothing, nada. NX is always described as a "console".