Did she? Mesaana fell, but all she was doing was causing havoc in the Tower. I have yet to see any evidence that the Tower made much of a contribution. Aes Sedai, sure. Female channelers, sure. The Tower, as an organization and institution? Nope. Even if Egwene is genuinely responsible for everything she is credited with, her agenda and accomplishments are all geared around the preservation of a moribund institution that has done more harm than good. The Tower is like a man who has hoarded a critical resource, prevented it from being exploited to its greatest benefit, and then demands assistance because everyone else will be screwed without that resource he has kept out of their hands. Throughout the series, everyone has done much better than the Tower. Their advantages in channeling over the Sea Folk & Wise Ones are due to less pressure enabling them to preserve a more broad array of knowledge, though that was none too broad itself. And most Egwene fans are full of high dudgeon and defend her criticism of Nynaeve and Elayne for making a bargain with an ancillary benefit being the dissemination of that hoarded knowledge, to women who are better channelers. The Sea Folk and Wise Ones are like marksmen who have had to perfect their skills and get their kills with antiquated or obsolete weapons, while the Aes Sedai carelessly flaunt their machine guns, which they master only enough to maintain their security over the unarmed. We need to get those machine guns into the hands of the marksmen and huntresses, rather than the bureaucrats. That's why they have all those skills like the Windfinder facility with weaves that enabled Talaan to beat Nynaeve with skill, when most Aes Sedai could not shield a woman so matched in strength. They were also noted as loosing fireballs at the Seanchan raken faster than the sisters, because they didn't need to move their arms to make the weaves. Aviendha shared that quality, and Sorilea criticized Aes Sedai ability as being impeded by their reliance on gestures. Where Cadsuane's attitude in that conversation indicates the Aes Sedai would not bother with the effort to overcome the handicaps of learning a second way, the Aiel apparently strive to do it the best, and to correct the faults of channelers, rather than shrugging them off. Their willingness to experiment and overcome difficulties through effort and practice allow them to perform feats like unraveling gateways.

Egwene saving the Tower, if that's what she did, only preserved the obstruction to superior channeling practices becoming dominant. Her theoretical feat is only important, because of the Tower's unwarranted dominance, that all but holds the world hostage. At the time of her plotline, we assumed that the social position of the Tower was necessary or critical to getting Rand's agenda through, but the only contribution the Tower made institutionally was by being manipulated by Rand.

As far as channelers went, there was no special Tower knowledge that made them more effective or gave them greater contributions to the Last Battle. Rand had the entire Windfinder population of the Sea Folk, he had the Aiel who outnumbered the Tower by a factor of six, he had his own sworn Aes Sedai to add a patina of legitimacy if it was ever really needed in the end, and he had the Black Tower. Most contributions made by actual Aes Sedai were without Egwene's input, and in spite of the Tower, rather than through it.

And was it really Egwene who did the saving? Because IIRC, the sine qua non of her win was Perrin's coincidental arrival with the dreamspike. She had a shitty plan, the Wise Ones and Perrin made a mockery of her supposed expertise in Tel'Aran'Rhiod, and her victory was over one of the least impressive Forsaken, because they were both stupid enough to think that Egwene being the Amyrlin Seat mattered. I'm pretty sure that if Mesaana actually became an nuisance to anyone who mattered, they'd have swatted her in short order.

The "Tower" was in no danger from the Seanchan. Rather, she handed the Seanchan their victory. According to the Seanchan leaders before sending the attack, the acquisition of Traveling would make the operation a success, even if their air force took 100% casualties in getting it. They did, only because Egwene's indefensible screw-ups caused Elaida and other sisters in the Tower to learn it. Beyond that, the Seanchan had no plans to destroy the Tower, only to make off with as many damane as they could carry away.
Egwene's best use is being expended.
By your measurement, Brandon Sanderson is the key to the literary success of the series, because he wrote the books at the end. Without him, the story would never have had a conclusion and been pointless.

It's a journey, not a destination. And even if you achieve a successful end, that doesn't mean everything you did was right. Successful people look at what they did and how they could do it better, that's how they stay successful. A major theme of the Last Battle, as relating to Rand's reincarnated status was "getting it right this time." You can't do that unless you know what went wrong last time.
By that's not true. They might have made mistakes and acted incorrectly, but they did not do so nearly as consistently as Egwene did, and for the most part, a lot of their issues have mitigating explanations. For Egwene, even her positive acts or effects have incriminating explanations or ramifications.
WoT is the core of our community. Unless we keep that alive, we won't be a thing anymore.