The fact that they think another Evolution won't be successful without Ronda shows how much faith they have in the women.
The first one was a success in spite of what WWE tried doing to sabotage it. I'm almost certain they wanted it to fail on the level of something like December to Dismember so nobody would talk about doing another one. But it didn't, and now their excuse for not doing Evolution II is "Ronda's not here. We need crossover stars to sell tickets.
"
The women's roster they have now is definitely capable of doing another PPV by themselves. WWE just doesn't want to try it again.