Serena, at her peak now, would beat a couple of the top 400. She's the best she's ever been and her serve, the best female serve ever, is around the pace and more accurate than most mens'... despite that, she'd struggle immensely.
It's not being sexist, it's merely telling the truth. Women are inferior because they simply do not have the strength or speed. She even said herself that if she were to play an exhibition against Murray she'd have to be able to use the doubles lines, have all of the service games and have Murray not have legs to even stand a chance... she knows she's nowhere near.
It's a shame that women aren't close, but that's why you don't judge them against the opposite sex....
Serena, to me, is the GOAT. Has absolutely everything.
The prize money issue is difficult. Even Billie Jean King, the most vital woman for equality in Tennis, argues that she understands that people can complain now, but that men should have a best of 3, rather than 5, game... as then more would be able to compete in doubles and have longer careers. It boils down to men wanting to continue with best of 5 due to the occasional mammoth game and tradition, and women saying "we can't handle best of 5, so reduce it to best of 3 if you're going to complain."
It works both ways. If men will complain without wanting to budge then it's their problem. Women, on the other hand, don't really have the capability to play for that much longer.