You have to connect the dots here. Dragons are basically immortal spirits who don't see time as linear. Mortal Finite Temporary are pretty much curse words to the Altmer, and that's what [censored]s with the Dragons so much. Dragons are et'ada. Altmer want to be et'ada. The tigers want to be "dragons."
The Altmer may not call them dragons, but they have the same goal. They want to be immortal, infinite, and eternal. To do this, they need to wipe out Man. Tsaesci are the Akaviri version of Men, whether they look like snakes or not doesn't matter here. Men cannot exist in any shape or form for them to get achieve divinity. Tamriel's Men need to die, and the Tsaesci need to die.
Curious how off-topic we are now, but also awesome.
As far as I know, the Tsaesci were the original persecutors. They are the ones who devoured the men of Akavir and they then hunted the dragons to extinction. We can't be sure of how the Po'Tun understand dragons either. They allied with dragons when dragons were a physical and present force, the metaphysics of everything may be may not be understood at all, or they may be understood in a way which could alter Tamriel's perspective of existence.
Beyond that, Tosh Raka's idea of dominance is not confined to destroying the Tsaesci, he does have eventual plans concerning an invasion of Tamriel. How deep is his understanding? And having achieved his own ascension, what exactly are his aspirations?
As an aside, I wonder if Akavir or further continents contain any Towers...
... I can has Akavir DLC?