Less diversity decreases competition and overall technological development. In the 1980s, some OS developers actually blamed Unix for stalling the progress of OS research.
Wouldn't a world of Arm based synthesizer be kind of stagnant? Sure you can dream that compatibility allows more open platforms, but we all know hardware companies always end up making the end product proprietary anyway.
To be fair, I don't know a lot about hardware. I'd also like to guess that the world of music electronics has been longing for some sort hardware medium to sort out the mess of obscure ISA's and poor documentation that hardware engineers have to put up with.