If, for one of the targets, you cant choose a new legal target, then it remains unchanged (even if the current target is illegal)
My prediction may have missed the mark, but it wasnt by much, as sapphire lands itself in the Top 3, just behind amethyst and ruby
A creature with both abilities will stop flying creatures and your opponent cant get rid of it with spells or abilities
Rather than aiming to take out opponents as quickly as possible, Control decks aim to 'control' the board, depriving opponents of key resources through removal and counter spells, preventing opponents from assembling their win-conditions while assembling a win-condition of your own
Usually the design team will pick what they think will work best and then playtest it
This thing also works well with the many exile and cast mechanics like suspend and foretell and adventure