Nobody.
Nobody needs 1000 HP to beat hardmode bosses, as they're designed around being challenging for players who have the maximum HP which can be reached at the time at which the bossfight can occur (except Eye of Cthulu, Skeletron and Eater of Worlds/Brain of Cthulu, which are designed around players with 200HP, which is why I support
this thread with all my heart crystals). For hardmode bosses, this time comes around the 400HP mark, 500 for Plantera onwards.
Now, some people might complain about the bosses being too difficult, them taking too much damage, etc, etc. But players shouldn't forget that Ironskin and Lifeforce are potions which are extremely useful in fights one might struggle with. Lifeforce increases the maximum HP to a whopping 600! It makes, for example, a first Plantera kill so much easier.
Say, if we implimented 600 HP being a pretty standard thing in pre-hardmode. Like, players will have 600 HP when fighting the wall of flesh. Inevitably, the Wall itself is going to be buffed as well. To compensate for this buff to the player, enemies will deal roughly 2x their previous damage, to not make the game boring, like it would be if the player never drops below 200HP or the Wall had as much HP as Skeletron Prime now has. It'd just be tedious.
With 1.3 and new, stronger enemies coming along, I can see the developers adding a new way of increasing the max HP to something along the lines of 550. Be it rare drops from some Martian Madness enemies, or an item sold by a new or old NPC; I can't predict that. This slight increase would tightly fit the player to the difficulty which is presented to them, while not making them too strong, as 1000HP certainly would.