If you had time to repair all the damage it would be fine, probably. But those cells that died are releasing all sorts of messages to neighbouring cells. It's not just that they died and now there is a hole to fill, it's that now there is a hole and neighbouring cells are also freaking out and not doing their job. And then if you have living organism the immune response is going to kick in and just go crazy at site that is damaged. You get the whole cascade when cells are all dependent.

Not necessarily, some structures never regenerate and scar tissue forms where the damage happened, so that part of the tissue never recovers it's function. That's why damage to some organs cannot be repaired.