You may be comforted by the fact that the top three people on the Github contributor graph[1] are not from those two companies. I've skimmed some of the published meeting minutes[2], and JCJ (Mozilla) and JeffH (Paypal) seem to be highly involved.

[1]: https://github.com/w3c/webauthn/graphs/contributors

[2]: https://www.w3.org/blog/webauthn/2018/01/11/meeting-minutes-...

EDIT: add forgotten link