Oh Serena is among the all time greats and in that too among very top, no doubt about that but I think the competitors are not a great match. Particularly in last few years since JHH retired. Navratilova had a great rival in Chris Evert. Also having watched Graf's dominant era, I think that era too offered better competition than current one. We see so many female tennis players appear and fade away currently. Even without Serena, none of them looks to be good enough that they will establish themselves as the top ones. Without Serena, still there will be lots of 'shock' results among other top seeds in female tennis. Just the sheer number of unforced errors in women's game is a turn off. Can't blame Serena for it though, you can only beat what is in front of you.
It will be great if someday women tennis too get a great competitive era like men's right now where all top ones are absolute class.