No—in fact, this move is both fair and exceedingly shrewd
First of all, it’s easy to see why someone would want to make such a move on purely egalitarian grounds. Why not grant the same opportunities to each species, to whatever extent possible? That very principle is the foundation of many modern societies. Tavi can’t make humans live as long as the Canim, but he can at least give everyone an even playing field as far as furycrafting is concerned.
Besides, I see no reason to conclude from the text that someone’s furycrafting can continue growing without bound for centuries. There’s probably still a limit; it’s just that now everyone can theoretically reach it.
Finally, I think it’s implied that, until the vord invasion, the Canim really did always have the numbers to conquer Alera, but simply didn’t do it due to internal reasons. Their continent was enormous and had huge cities. So what else is new, even if the Canim can become more powerful?
But there are certainly very practical reasons to support such a plan.
Quid Pro Quo
Tavi believes that if the various races of Carna share what they know with each other, all will benefit. He believes that the Canim have something to give humans, as the Marat did.
“I cannot say the same,” Alera responded. “For which I find myself…
somewhat grateful. It would be awkward.” She inhaled slowly, then
nodded. “Are you sure you wish to continue on the path you have
begun?”
“Well. You say I introduced you to Kitai, without realizing it,
because of our bond. That’s why you can speak to her.”
“Indeed.”
“Then you should trust me. Interaction with the other Marat will be
just as rewarding, on some level. As it will with the Canim. And the
Icemen are already watercrafting, whether they realize it or not. It’s
hardly any change at all.”
He’s probably right, too: the Canim and Icemen have their own sorcery, which the humans can benefit from learning. As with trade in the real world, though some may suffer, and certainly some will benefit disproportionately, there’s generally a net benefit to both parties. And links with the Marat offer their own possibilities.
Who wants to die?
The Vord are going to come to Alera. As Alera herself says:
“You have a century and a half, by my estimation. Perhaps two. And
then the Canean vord queen will come for you.”
Tavi nodded. “Then I’ll make us ready. Or get us part of the way
there, at least.”
The Canem vord queen won’t have the furycrafting prowess of her counterpart, but she’ll have numbers beyond anything Alera has seen before. The Canem continent and its resources were enormous.
When an existential threat is knocking on your door, do you want more furycrafters, or fewer? It’s what is colloquially called “a no-brainer.”
Besides, if the humans give their sorcery to the Canim and others, they buy goodwill, and (in keeping with the above points), are more likely to be granted the Canim and Icemen and Marat’s secrets.
And then everyone wins. Or at least doesn’t get assimilated by the vord. And honestly, that’s what they’re aiming for at this point.