I think the word luxury has two tiers:
Exclusivity. A product that is sought after but accessible only by some very small fraction of the population for example by status or extreme wealth.
Imitation. A product that is not exclusive, but is marketed as sharing features with an exclusive product to improve its popularity. It is only somewhat exclusive by being slightly more expensive.
For example the American Express Centurion card can be considered a tier one luxury credit card because it is not only very expensive, but also is by invitation only.
The American Express Platinum and all the other imitation Black cards are packaged products to give people outside this exclusive club a taste of what it's like to be in the exclusive club without actually being in it.
I think this is why luxury cereals aren't common. It's not easy to establish exclusivity around this product and without an establish exclusive product already existing it is difficult create tier two luxury cereals because there is nothing to imitate.