it’s not really a question about the past… rather as a current product, why not move to it?
(a direction has been id say fairly clear for quite a while now)
not necessarily, since you have to have a driver on your host computer anyway, there is nothing stopping that driver also supporting the midi protocol, and then translating it to the existing protocol for backwards compatibility.
hmm, a mobile OS thats extremely popular for music software 
besides its not just about iOS is it?
how many threads are there for support issue with serialosc, or asking about if it currently serialosc support is currently supported on the latest version of an OS?
the reason many manufactures have moved to USB class compliance (generally, not just midi) is it moves part of the burden to the OS developers.
the product developer just needs to concentrate on compliance to the standards, once thats done they dont have to worry too much about new OS versions etc.
this is also why end-users prefer it, since it means they dont need to worry if the product owner will update their drivers for new OS … which given the trend to OSs updating frequently this is a good thing.
Im not saying usb class compliancy has to be for everything e.g. the Eigenharp could not use it, you’d not get the throughput required, but the grid and arc have reasonably limited interfaces and data rates, so seem like good candidates.