Without Serena it would go on fine with the other champions in competition. It's not because she didn't find a rival able to handle her that the game won't be exciting anymore. We're still excited with many other matches without Serena.
It's not the level of the game itself that makes it exciting, it's how both levels of game match up. In some way, when a player, whether it's Serena, Graf or Navratilova in the past, dominates the opponent outrageously, I'm just bored. When Graf was dominating the field, it was boring. It was only exciting when she had opponents able to challenge her. Same for anyone else.
Fortunately, we had players able to challenge Serena pretty fine this year and the previous ones. Azarenka comes pretty close, especially in the fighting spirit department, and actually Vika is better than Serena in the rallies. No kidding. Serena has to raise amazing striking ability to get Vika (that's what she did at Wimbledon).
Safarova, Halep have been challenging her this year. Kvitova has beaten her in Madrid (that wasn't Serena at her best, but hey, it was Serena). We know how tennis matches go, and if Serena is clearly the better player, her dominance isn't so huge. When you recall how some other champions were dominating their era with double bagels or double breadsticks, we cannot really say that's the case today.
To me, the field is great.
As for who I see as the next big one... I'd go with Stephens. She's improving slowly, but her potential is huge. But I don't think any champion will have a great dominance, and these eras are my favorite. Multiple champions fighting at the top is more exciting than just two or one dominating the others.
Actually you have to give credit to Serena for being dominant in a strong and challenging era.
On the paper, you'd say the top 10 in 2001 was amazing and stronger: I'm not so sure. With the way Serena plays today, I'm not sure Venus, Justine, Kim, Martina Hingis, Jennifer, Lindsay would have got a say. Not sure they would have won slams either. We will never know anyway.
As you say, there are good players in the field that can challenge (at times), but consistency is a huge issue for me. As good a player as someone like Safarova is, I just don't see her as a GrandSlam champion. Maybe the next big one is Muguruza, who is making steady progress year on year.
I guess what I'm saying is, who is going to be the next GOAT? The line of succession that I can see is:
Hingis ( slightly transitional)
Venus/Serena, with Justine thrown in.
Everyone else during those periods to me are just supporting cast.
In each of these cases, previous champions have been taken over by new champions who naturally took over and started to dominate the game. I just don't see this happening, and can't help feel that when Serena leaves that we'll be left with a bunch of also-rans. We need someone to step up and start beating Serena with some degree of regularity as has happened in the past line of succession, so that there would be some level of legitimacy to the next in line. I think without this, the next no.1 won't have an air of credibility.
I'm not arguing whether the standard is better or not. I know we have different opinions on this. My argument centres on whether the next champion makes Serena think that her time is up because she is being consistently beaten, as all of the champions before her were. At this point in time Serena is chasing and sweeping all the records before her because she is just so much better than everyone else.