I would agree Gyarados should have been typed as a Water/Dragon Pokémon from the start. A change in its typing would have been different that the change in Magnemite and Magneton though. Reason being is because they had a type added to their original place rather than anything being altered completely. However, the only reason why so many people consider that it should be represented as a Dragon subtype is because of its look and origin. Like explained, Gyarados derives from the Chinese belief of an honored Dragon myth, however this is Pokémon not reality. So the choice they made for it to be a Water/Flying rather that a Water/Dragon is only because this is Pokémon and not realistic belief.
Gyarados wouldn't have been the most strongest Pokémon in the game even if it were to be classified as a Dragon type though since the first generation. Yes, its stats are incredibly well and comparably different from Kingdra. Although, that isn't to say a normal effective move and an even stronger Pokémon couldn't have defeated it. So, this thought really isn't effective like how you've mentioned it to be.
Like mentioned, making him a Water/Dragon wouldn't have made him a Pokémonopoly. Overtime Pokémon changed, and even stronger Pokémon came to be. If it were to be a Pokémonopoly as a Water/Dragon then it might have been classed as a legend such as Arceus, but really with only one Dragon move at that time, it wouldn't have added up like it would have now. So, he would have still has his downfall regardless of the typing.
Lastly, nothing in the game would have been imbalanced regardless of Gyarados being a Dragon subtype. It would have just been different. With 493 Pokémon so far, and possibly more to come, Pokémon still hasn't become imbalanced and if Gyarados were to be a Dragon type since the beginning then nothing would be been off balance, so your use for imbalance to the game doesn't add up for me, sorry. :\
Overall, Gyarados is it was it, not a Water/Dragon, but a Water/Flying. Sadly, it's only flying move available to itself is Bounce, but regardless he still has better moves to cover up for its loss. It is still a highly used Pokémon with great history. If it would have been a Dragon type, then yes it would have been strong, but it wouldn't be the same Gyarados we were to know today. Basically, it is what it is no less of its look-a-like dragon appearance or its moves.
Gyarados wouldn't have been the most strongest Pokémon in the game even if it were to be classified as a Dragon type though since the first generation. Yes, its stats are incredibly well and comparably different from Kingdra. Although, that isn't to say a normal effective move and an even stronger Pokémon couldn't have defeated it. So, this thought really isn't effective like how you've mentioned it to be.
Like mentioned, making him a Water/Dragon wouldn't have made him a Pokémonopoly. Overtime Pokémon changed, and even stronger Pokémon came to be. If it were to be a Pokémonopoly as a Water/Dragon then it might have been classed as a legend such as Arceus, but really with only one Dragon move at that time, it wouldn't have added up like it would have now. So, he would have still has his downfall regardless of the typing.
Lastly, nothing in the game would have been imbalanced regardless of Gyarados being a Dragon subtype. It would have just been different. With 493 Pokémon so far, and possibly more to come, Pokémon still hasn't become imbalanced and if Gyarados were to be a Dragon type since the beginning then nothing would be been off balance, so your use for imbalance to the game doesn't add up for me, sorry. :\
Overall, Gyarados is it was it, not a Water/Dragon, but a Water/Flying. Sadly, it's only flying move available to itself is Bounce, but regardless he still has better moves to cover up for its loss. It is still a highly used Pokémon with great history. If it would have been a Dragon type, then yes it would have been strong, but it wouldn't be the same Gyarados we were to know today. Basically, it is what it is no less of its look-a-like dragon appearance or its moves.