True, someone aleready experimented a multi algo coin, i forgot the name of the coin, but it can't bring good results as each algorithm requires custom GPU tweaks, so if you change the algo at each new block, miners will leave. Annother sollution would be to define an ASIC safe algo and use it, i don't know much about Keccac, but if luke-jr believes that he can be amended to fit in bitcoin-core code, then we should go for this sollution.
Fair concern ... but if the algos match ETC and zcash and merged mined couldn't this concern be negated as we would be bringing in 2 different communities to help secure Bitcoin? ETC and zcash folks get along with Bitcoin Core folks as well and share many similar values and we could benefit from their collaboration against those who wish to attack immutability.