As I have seen, with Serena at the very top leading them, the others will work harder, play better with the real Everest target to aim for and climb up to, but will win less. Conversely whenever Serena is out for a while, as in 2011 or 2017, the women, without their pack driver, will drop off after a bit, be more inconsistent, injured, and the trophies will get scattered around more. I regret to say that for a number one, Halep was not very impressive. I do wish she had done better game level wise in 2017. She was playing better when she wasn't number one.
For example, Azarenka worked really hard, tried incredibly hard in 2012, 2013. She was so fit and playing so well, she was able to almost reach Serena's level for a little while, she was the only one (I should count Kerber as well) in the past 10 or so years to come close, yet she was still held down by an even stronger hand. imo, an Azarenka playing at her '12, '13 level in 2017 absent Serena would have won nearly everything, definitely French and US. I'd say that 2012 Azarenka on her form then would certainly have swept all four of the RG and US finalists, Halep, Ostapenko, Keys and Sloane. They haven't been anywhere as good as 2012 Aza. Maybe Wimbledon Aza would have won as well. I don't know what her form is like now or how fit she is.