I can't imagine Monica fast enough to deal with the sheer velocity of the groundstrokes of a Venus or Serena or Justine on grass. Or even Capriati or Petrova. The ball gets through the court so quickly. Monica needs a surface that slows the ball down enough for her to catch up to it.
As for why Lindsay and not Monica?
Lindsay's in way better physical condition, and when she won it, she had by far the biggest serve on the tour. And one aspect of the tour that has really changed since Lindsay won (BECAUSE Lindsay won) is that return-of-serve has gotten better on the tour. There are many more players who can return a 100+ mph serve with an outright winner. Serving used to mean automatic control of the point. No longer true. Monica has to be ready to move, move, move.
And, correct me if I'm wrong, but Monica had fractures in her feet. That's a weight-bearing issue. Put bluntly, she can't get away with being fat. If she's heavy, every running step put an incredible load on her feet. If she's out there at 175-180 lbs again, she'll just break her feet.
She doesn't have to be in the shape that Serena's in. (I mean, who is?) But I think she does have to be in the shape that Lindsay's in. If you want it in comparitive weight terms, she has to weigh less than Venus.
Proud to be an American
Not blind. Not uninformed. We are party to atrocities. But the response of the world after 9/11 is worth noting. Even our most dire enemies offered aid. We should all be so lucky.