I love Ellen Pompeo as Meredith Grey and she will be missed on the show. But so long as the writers keep to the same format, bring in new interns for Lexie and the rest of her class now that they are residents and start focussing on new characters/interns.
At the very beginning of the series Chief Webber gives a speech basically telling the audience that the show/series will run for 7 years. So we'll have Cristina, Alex, Meredith and Izzie all either leaving once their 7 year surgical residanacy is over, they will then be attendings ready to torture/teach new interns, maybe have flashbacks to when they were interns, and see how much they have grown as surgeons. There is room for the series to go on long after the main characters have left, that's why we have seen Seattle Grace and Mercy West merge, so when main characters leave they can focus on the new characters and watch there friendships develop.