Well.. you already see the multifunction capability (Spectrum Analyzer, 1 direction VNA) in popular instruments like the Keysight Fieldfox, and similar devices from Anritsu, etc.
There will always be a market for single purpose devices - highly cost (or size/weight/power) constrained applications where you don't want to spend the extra pennies for the components; and where you need the ultimate in measurement performance. It's like counters and spectrum analyzers - both can measure the frequency of a signal, but they tend to have different reference oscillators (ADEV vs Phase Noise optimization) and because they use different measurement techniques, they are optimized for particular applications. Another example might be precision measurement of RF power - a broadband calorimetric power meter or a narrow band filter with a detector (what a spectrum analyzer is). Getting 0.5% accuracy from a broadband power meter is straightforward, getting 0.1% uncertainty is difficult, but possible. I don't think there's a spectrum analyzer made that has that kind of accuracy. Typically, it's more like 0.2 or 0.5 dB (5% - 12%).
