No, you're just defining away a weak era by saying anytime Serena loses, that player must be really good so by definition, if Serena loses, it can't be a weak era. We know it was a weak era because flash-in-the-pan players won majors. In strong eras, that almost never happens. For example, from 1998-2007, every major was won by a strong established player except one. Iva Majoli was the only outlier. Since then, you've had plenty of flash-in-the-pan winners from Schiavone, Stosur, and Bartoli to Pennetta, Ostapenko, and Stephens. Compare that to the men's side. Since 2005, there have been no outlier slam winners except Cilic, and he's more consistent than any of the women I just named.
None of them are even as consistent as Sabatini, much less Serena.
No. If you think everyone is great (like Schiavone, Stosur, Bartoli, Pennetta, Ostapenko, and Stephens), that just means no one is great. Greatness in sports always means rising above most everyone else. Wozniacki just retired. Her entire generation failed to produce a single all-time great player. That's never happened before in the history of tennis. You always had a Court, King or Evert, Navratilova or Graf, Seles or Williams, Henin until 2008. In the Open Era, the men's side only had a lull like that between Sampras and Federer around 2002-2003. The women have had it for over 10 years. Graf, Navratilova, and Evert had tougher opponents in one another than Serena ever has except maybe Henin and relatively briefly, Venus, but again Henin retired 12 years ago and Venus has been a shell of herself for about that long.
I personally think the weak era is coming to an end. I think if Osaka can get her mental game together and Andreescu can stay free of injury and Barty can bring her A-game more often, any of them could become the next great player.