Those who are already paying for AD will most likely not swap to...

Those who are already paying for AD will most likely not swap to KiCAD, no danger there. The point is the seats that KiCAD can take away from AD for new customers as the pipeline of new engineers come through that have had experience with KiCAD as it evolves over time and have modest needs.
Here is a list of projects using KiCAD, many of which would have ordinarily opted for something like Altium. Some things as complicated as laptops.
https://kicad-pcb.org/made-with-kicad/
https://www.olimex.com/Products/DIY-Laptop/

As for buying quality, Altium actually has a rich history of long standing issues, some of which have gone unresolved for almost a decade. Anyone who has ever used it for serious professional work will attest to this. Some versions were so bug-ridden that customers skipped those releases entirely, just like people skip Windows some releases.

I'm not saying it's doom and gloom for Altium, far from it, but KiCAD is not an insubstantial threat, so it's wise to keep an eye on it.