If one of them loses their peg for whatever reason it'll primarily be LPers in the d3 pool that are affected. There is potential for alUSD breaking peg in that case too but it seems unlikely.
We have mitigation of that risk already in the form of: The Transmuter, alUSD curve pool, and a 200% collateralization for alUSD. Unless the pool contains a HUGE amount of alUSD in the future it wouldn't represent a major threat imo.
If, as an example, Frax as a protocol just falls over dead - We'd poke it with a stick, call an ambulance, send flowers, pour out our drinks and carry on with Alchemix business.
