Decades back there were at least half a dozen instrumentation companies offering Dynamic Signal Analyzers (DSA's). Not Spectrum Analyzers, but rather Dynamic Signal Analyzers. The main differences being dual or multiple channel input and frequency ranges that went down to milliHertz. The DSA's were complete systems that would allow one to measure the transfer function of a servo system and analyze its stability margins in a matter of minutes.
I recall HP (Agilent), Onno Sokki, Bruel & Kjaer, Anritsu, and there were at least three others. But today there is only Keysight (Agilent spinoff) and Stanford Research System's 780 & 785, but the Keysight analyzers are the only viable ones for any serious servo work.
So where did this market go? Surely the demand has only gotten bigger with the rise of robotics work. I'm somewhat dismayed that development of new analyzers has not advanced beyond the HP35670A, which I consider the best DSA ever designed. Keysight still offers that model but the inputs are still analog. And no USB to get the data off.