As an early advocate of subs, I still have a weak spot in my heart for the biz model. I think there’s still a substantial potential customer base for them, especially if they include (a) unlimited DRM-free downloads (which UMG has already gotten behind), and (b) streaming to mobile devices, with caching, a la Spotify. Obviously, the market size won’t be anywhere near $40B global (record retail at its peak), especially in the early years of deployment, so they need to be integrated with other revenue models, from ad-based to licensing, and supported by 360 artist management.
All that being said, your math is off.
25mm subs x $10/month is actually $3b per year, not $250mm. Which is a pretty nice chunk o’ change. And 100mm people at $20/month (which I believe is an unfeasible price) would actually deliver $24B in annual revenues.