I'm looking at wikipedia, combined with around 40 years of being a fan of both bands.

Please look at both "timeline" and "lineup" for reference. Fleetwood Mac had two singers from go. You prefer a revision of that accounting to suggest that one of the singers wasn't involved in order to arrive at your premise.

Secondly, band A used a combination of two names, where band B used an existing name.

Further, you want to imagine that Van Halen was a totally different sounding band under Hagar than under Roth, which is absurd... Nobody at all has ever been confused about the band they were hearing, regardless of Roth or Hagar.

Fleetwood Mac's changes were profound, and the majority of their current fanbase would not identify Green era Mac as Fleetwood Mac.

There's more, but again... accuracy matters more than a flawed conversation.