Most of the time, it seems more sarcastic than serious, unless you missed the bit about dolphins being killable.
And why do sharks HAVE to be a common waterborne hostile mob? There's more original options, such as jellyfish, dolphins (After all, dolphins are among the few animals that kill for fun, and it would be easy to make hostile dolphins look like they're from a separate species), stingrays (Specially if Mojang want to make griefing swimming pools more fun), sea snakes, electric eels, moray eels, coelacanths, crustaceans of unusual size... There's a lot of options, even if only RL animals are considered (Specially as the game already has comparatively obscure flowers and Ocelots, which are pretty rare in fiction, which means that obscure animals can be used as well), to the point that using sharks as a hostile water mob would feel almost painfully uninspired (Specially coming from a game where ocelots can scare away human-sized walking exploding shrubs, there's flying fireball-shooting jellyfish, and there's an entire mushroom-themed biome).
The "reasoning" was most likely a sarcastic response. They most likely won't be rideable, but not due to some weird concerns.
Parrots can detect nearby mobs, practically acting as pseudo-short-range radars.
Alright, makes sense.
Also, all of these ideas you offered aren't any better than sharks, mainly because they're all real-life animals. To be honest, sharks would make a terrible choice for a common ocean hostile mob for that same exact reason; I just think they'd be good to add because, well, we have neutral predators like wolves and bears, so why not have neutral sharks?
Stuff like Mob A or other mythical/made-up sea monsters would be really good as hostile sea monsters.