Most of those concern may not apply if a third party releases an add-on by their own intent. If there's a chance to make a profit purely out of selling the hardware, a third party can be interested to release a product under their own brand with some or little help from Microsoft.
In any case, it's hard not to imagine Xbox 720 doesn't have a Blu-ray drive in it, the broadband future is a bit too far to make it in 2011. Some of the concerns above are minor issues in a long-term view.
But there's another possibility. It looks like this:
1) Microsoft releases a new Xbox 360 model with an internal BD drive, plus a motion controller or other enhancements. (It can even be more powerful hardware-wise than the current 360, but it's another rumor)
2) Microsoft also starts to release games on BD, such as Halo 4.
3) Those same games become available on multiple DVD sets too, but those require HDD installation.
If it's Halo 4, Core/Arcade users will be compelled to buy an HDD drive. After the platform holder shows the new scheme can actually work, third parties can follow.
All those things depend on how Microsoft thinks of the Xbox 360 plan these days, the console architecture which was supposed to last a lot longer than the first Xbox to recoup their investment.