Agreed. You can go for a sovereign with all the spell schools if you want to. Taking only fire is a calculated risk you decide upon.
Actually its not a calculated risk, because the player does not know how high the chances are that a fire shard is close to his starting position and there could be no fire shard on the complete map. It would be a calculated risk if the player knows that there is chance of 50 % to get no fire shard, a chance of 25 % to get one fire shard and a chance of 25 % to get two fire shards.
You know you have access to about 2 of 6 shards near your starting location. If you are focusing on magic, you'll want at least 4 elements for your Sov. But you will [probably] also find mages throughout the game. Many of them will have Fire magic. It's not that crucial. Magic Sov builds are not risky. Only taking Fire Apprentice is risky.
I have underlined the random parts and added the word probably to show the high randomness in FE.
