The turtles will evolve to fit all other niches
And eventually, they will stop being turtles.
I´m going to mention here the book "After Man, A Zoology of the future" (Dougal Dixon, 1981).
Among all the topics mentioned in the book, I want to mention this one: In a few centuries, a new volcanic island emerges. Brand new with no life at all. After a few months, the first living organisms in the island are plants. And later,the first living animals arriving to the island are bats. (And here comes the answer to the turtle part). Dougal Dixon explains: since bats are the first animal species arriving to the island, they become the dominant specie, and evolve and start occuping many types of niches. For example, some loose their wings and adapt to a ground life:
Others bats change more dramatically and adapt to acuatic life:
The point is: If all the animals disappeared except the turtles, then the turtles will evolve to fill the empty niches, and in a few milenia, you will have plenty of animals that will have very little (if any) resemblance with the original turtles. You could expect to have predator turtles, turtles that run, turtles that climb trees, and (perhaps) even turtles flying. The only way a turtle could mantain its turtle style, is if the conditions in the new ecosystem give the original turtle an advantage versus transforming itself in other type of organism.