I have experimented with different ARM based SBC systems and boards.
One thing I had to learn the hard way is: don’t underestimate the community. Doubtlessly, the Raspi is almost 10 years on the market, extremely widespread and kind of set the standard.
I used some very interesting SBC with sometimes more power than the Raspi version of the current time, but such boards get relatively useless if the campaing / small company just disappears or is far behind the promised support planes. Still there might be support of the SoC manufactorer itself, but updating kernel and BSP and that stuff is time consuming and something you do not want to care about.
Fully Open Hardware is definitley the way to go in the future but the plattform needs widespread acceptance so that “workload” can be spread and projects like Zynthian eg. can rely on an up-to-date kernel and linux system. Booting the SoC and graphics unit (sometimes also video encoder/decoder units) are still a major pitfall for many ARM based systems - although the situation improved over the past years. In the past, some manufacturer provided “open” video drivers, which meant some kernel module which loaded a BLOB (kind of binary firmware) and of course, the only provided this for certain and few kernel versions…
I am curious which platfform is going to solve this best - hopefully in the next years. Maybe some ARM system, maybe also some x86 Atom with an open UEFI implementation and good mainline Linux support, maybe something different. Consequently, we should stay open and monitor new and interesting options. Though, I would not drop the Raspi easily
And regarding the I2S discussion. IMHO the more powerful the SoC / board is, the less important this becomes. Just use a decent powerful but cheap microcontroller. You get USB 2.0 and for sure enough UART, SPI and I2S - that’s it. Choose something like arduino or teensy with good library also regarding USB and audio and make it act as a class compliant USB device which features audio and MIDI. That’s it and you won’t have to worry about drivers at all and can use it on any modern system. Audio hats could still be used, since I2S is a standard and it would just mean connecting the right pins and assuring GPIO levels are the same.