I think this a fair point, but misses part of the argument--I don't think the Serena side is saying that Serena consistently dominates others day in, day out every match like Graf did, rather than that at their peak level, Serena def. Graf. (In other words, Graf's peak can last longer/be more consistent and Serena's may be only lasting a oouple matches, but Serena's potential level > Graf in a best 2 out of 3 match.
If that is the argument then I don't see how that makes Serena the "GOAT". I agree, Serena at her best would be hard to beat...but like you said Serena was rarely at her best, and rarely even NEAR her best...She has improved upon that in the the last 2 seasons, but for the rest of her career she was either great or off.
To me, and I'd imagine most people, you would want the best player to not just be the best on rare occasions. Graf vs Serena on 10 random days throughout their career would end in Graf's favor, because it would be highly unlikely Serena would be anywhere near her best in the majority of the matches, whereas Graf was consistently great.
Picture it this way---You have a player, let's say similar to Mary Pierce, who can just wake up one day and be untouchable. However, she rarely does so and only wins a couple grand slams. You wouldn't consider this player in the conversation for GOAT.
Or even think of a player like Sorana Cirstea. Cirstea has been phenomenal on VERY rare occasions. She likely could take down a few of the top ten players on those days. Yet no one would consider Sorana a better player than Radwanska.
What Serena proponents are missing when they claim her to be the sole "GOAT" is how huge of a role consistency should play. Serena is celebrating winning her 600th victory. She has 110 losses. Graf had just over 900 victories and only 115 losses. You can make the depth argument that tennis is more available in many different countries, but I counter that argument by saying that other women's sports are also far more popular than they were in the 70s, 80s, and 90s, and tennis might have even lost some popularity, which one certainly could say would take away from the talent/depth.
Her longevity is also questionable. Her ~710 career matches is less than half the total of what Navratilova WON (1442). I know the game is more physical now, but really? We all can agree that Serena played large portions of her career out of shape. The lack of total matches is due largely in part to that.
Serena certainly deserves to be in hallowed and rare company as A GOAT, but I can't see the argument for THE GOAT. Be content to have her name etched in history alongside Navratilova, Evert, Court, Graf.