I think one of the issues identified by OP was the lack of prior advice that the A380 had gone unserviceable. It may have been decided while the OP was in transit to the airport but then again it may have been apparent for some hours.
If the latter, then totally unacceptable for a F pax (and WP1, WP for instance) not to be contacted and advised, even if only by text. Even if nothing can be done, I would expct this courtesy from the airline.
In cases like this (with an undeniable superior experience in A380 F Vs alternative), if the A380 is u/s with plenty of notice resulting in a metal downgrade, F (& WP1, or WP in any cabin) are pax entitled to re-book?