I'd say the smartwatch solution set, with the exception of Apple Watch is under-spec'd still, in terms of what you get, the technology is woefully short still, particularly in the performance vs battery life. It is obviously one of the more challenging areas but at least its not mechanical nature, so not so much reliability, but efficiency of electronics vs battery density, limited by the wrist form factor. Apple is expensive.
But the big non-folding phone is mature and good value, the "phablet" market there are multiple big phones out there which do not rely on mechanical movement.
Hence so long as your expectations of a smartwatch are kept low - use them sparingly - then a smartwatch + big phone / small tablet, is more viable than the folding phones, in 2019 at least. I see the smartwatches getting better faster than folding phones getting reliable so I expect my view valid for at least a year til the folding technologies get to version 3 reliability then version 5 cost.