https://github.com/stpeter gave me a heads-up about this.
tl;dr: All the same reasons for not re-using WebID apply to Web Sign-in (and Sign-on is too similar), and thus we object to the proposed re-use.
There’s already an existing set of related specs, numerous deployed & in-use implementations, and an open standards community actively using (including numerous actual users using) the term / phrase / technology.
https://GitHub.com/BigBlueHat wrote in https://github.com/WICG/WebID/issues/41#issuecomment-742737907:
> Naming is hard and taking an existing name from an existing community doesn't win you any friends or collaborators.
To state it even more strongly, Google of all parties must not act in a bullying way (we must consider the outsized influence & power dynamics), even within the auspices / context of a CG (using a vote in a CG to justify squatting over an existing active spec and a community’s use thereof). Rallying more folks to tacitly or otherwise approve of bullying is still bullying, perhaps even a worse form of doing so.
I can sympathize with the naming challenges in the area of identity (seems fitting).
That noted, an exploration in a CG seems premature to worry so much about a "marketable" name, especially in an area where naming is hard.
Instead, make up a throwaway placeholder name (like WID2021), first get the technology right, working across at least a few different vendors relying/consuming each others identities interoperably, and then worry about an actual marketable name, perhaps at WD/CR time. We know this can work per the prior example of "Atom" which went through a few throwaway names like "Pie" before being standardized as Atom in RFC 4287 at IETF.
Thanks for your consideration,
 https://indieauth.spec.indieweb.org/ (previously a W3C Note published by the Social Web Working group: https://www.w3.org/TR/2018/NOTE-indieauth-20180123/)