Is there a universal MIDI router/multiplexer box with a whole bunch of USB and MIDI ports on it? If not, would there be a market for such a thing?
Nope. You can do this by plugging a USB hub into a computer/tablet/phone, and running several MIDI interfaces into the hub, so it's not an unsolvable problem for the musician, just a very clunky setup.
Didn't find a single device with multiple USB and MIDI in/out ports after an extensive search. And the devices that I did find cost more than the synths that they would connect.
>If not, would there be a market for such a thing?
There is a market for a compact device like that, especially if it can be battery-powered for live gigs, remembers the settings, and doesn't need a screen/external device to set up the routing.
Even something as simple as "Merge all IN ports, and send to all OUT ports" would be great, and there isn't such a thing.
Seems like most musicians who care about this simply don't buy gear without 5-pin ports to begin with.
But yes, I've been thinking about making this for years now, and if you need a startup/kickstarter idea - go ahead, I'll chip in.
For old MIDI DIN you would need a MIDI adapter or use the raspberry pi serial ports and some circuitry. Can be powered by an USB powerbank for example.
https://github.com/davidmoreno/aseqrc
I also added to the mix rtpmidi (https://github.com/davidmoreno/rtpmidid), and USB MIDI gadget (https://github.com/davidmoreno/midiconfigfs) on the PI, so then I can use the pi itself from one or several computers via Ethernet and USB.
All the MIDI host boxes I've seen only have one USB port, so can't be used to connect two UDB-midi devices to each other.
It does solve all the problems I mentioned, and yes, it's not the simplest device (and sadly, I can't say the high cost is unjustified).