Since they were available in Ruby, Sapphire, and Emerald, they'll be obtainable in Omega Ruby and Alpha Sapphire as well. They like to keep remakes closely reassembling to their original versions, for most cases, so it wouldn't make much sense to be removing these Pokémon, if you ask me.
Though, what I believe would be nice is if they had made their locations (Birth Island, for example) accessible from in-game, without them distributing any ticket of sorts via Mystery Gift. It'd be very fun if we were able to discover places like this by simply exploring, whether it requires us to surf, dive, climb rocks, or anything we could possibly think of.
I'm sure a majority of fans would appreciate them giving us chance of having more adventure and whatnot, including me.