Think of the combined power of all those High Elven mages.. Think what dastardly spells they could think up to affect the ecosystem there, there's so many things they could do to force the Argonians to surrender. After all even the Dunmer managed to enslave the Argonians..
The Argonians are skilled in magic as well(if the games are to be used as an example, then Restoration, Alteration, and Illusion). Can the Altmer even mess up an ecosystem especially one as potent, and deadly as black marshes? As for the Dunmer enslaving the Argonians I imagine those were due to kidnappings, or backroom deals, and such.
I admit I'm unsure, but I assume when you said enslaving the other races you meant the Altmer would conquer them first. Also wouldn't the Hist Tree's do something if they thought the Altmer were a threat?