What's the long-term play for Canonical here?
Open source fundamentally is a do-ocracy (it's in literally all of the licenses). Those who do, decide; and more and more often those who do are just one or two people for a tool used by millions.
The obvious potential motivations are things like making a more reliable product, or making their employees more productive by giving them access to modern tools... I guess I could imagine preparing for some sort of compliance/legal/regulatory battle where it's important to move towards memory safe tooling but even there I rather imagine that microsoft is better placed to say that they are and any move on canonical's part would be defensive.
Presumably it's rewriting critical parsing code in APT to a memory-safe language.