This would make a great case study for migrating a full stack AWS app to Azure. They're constantly trying to convince their customers to migrate to full stack Azure, here's their chance to prove it's doable. Dog food and all, I'm sure they'll find some pain points they could fix.

Not doable. Most azure services are not "compliant" for o365. Especially not in gov clouds. Exchange has its own totally separate backed which existed before azure. The port of wonderlist is to exchange.

Still, moving product to new servers and improving security sounds to me easier than rewriting it.

It isn't just a change of server, it's an entirely different back-end infrastructure. To Do is a modern UI on top of Exchange tasks.

Speaks in part to how much vendor lock in there is with cloud providers that this has been such an involved migration.