Jimmy,
you are correct in your point relating to the 31,500ft restriction for older MSN departing with a bleed inop., departure should be with the x-bleed selector sw open. The APU can be left running for bleed air supply if required also.
There is an OEB for a bleed indicating abnormal pressure, this procedure provides steps to reduce pneumatic loading on that bleed and requires monitoring of the pre-cooler outlet temperature. This is an effort to protect your good bleed. However if I dispatched in single bleed config and saw problems developing with the second bleed i would be getting down quickly.
Of course if both bleeds fail, both packs will fault with no air to serve them, and the cabin starts climbing. There is then an abnormal fcom procedure (not directed by ecam) for AIR DUAL BLEED FAULT.
Bleed faults are caused by overpressure, overheat, wing or engine leak, valve not closed during engine start, bleed not closed with apu bleed on.