Tennis players, just like any other athlete, come and go. There are times when in their youth, they're young and hot and seemingly unstoppable. Others, come and make a big splash and zoom, they're gone. Others they stay longer than expected and become legends.
Maria Sharapova, in my opinion, has had her good years behind her. She was but a teenager and when she stared winning major titles and for a while she was a force to reckon with. Then the Williams sisters seems to unstoppable. Today, they're ridden with injuries and sometimes can't even make it to the quarters. When Mother Time gets to you, there's no stopping it.
Young and talented players keep appearing out of nowhere and making it to the pros. How long and how good will they be only remains to be seen. Maria still has a few more good playing years ahead of her and with some luck, might win another grand slam or two, but she will have to be very very lucky.