What happens to a Titan Shifter if they die without being eaten?
The answer to this question can be found in Chapter 88:


   
 


Or, to quote the wiki:


   If an individual with the power of the Titans dies before it can be transferred, the power will be passed on to the first Baby among the Subjects of Ymir, who is born directly after the said inheritor's death, regardless of distance or relation to the previous inheritor.




As you can see,


   as long as there are Eldians remaining in the AoT world, the power of the Titans will never vanish.