...this is not the NFL. We lose players every 4-5 years. How do we know if Tim Bailey and Brandon Cooper are going to be any better than Titus and Avery by the time they graduate? Obviously, anytime you lose a starter to graduation, the guy that is going to step in to fill his shoes is not going to come in and immediately be as good as the graduating player, otherwise they would've started over the graduating player. The hope is that they will be as good or better than the graduating player by the time they leave.
You can't compare a true freshman to senior, experienced player. We can look back and compare Jerious to JJ since their careers are done, but it is the definition of apples to oranges to say "Robert Elliott is or is not as good as Jerious Norwood." Nobody knows.
The only halfway meaningful comparison is the ridiculous system of recruiting rankings. Titus Brown was a 2* linebacker prospect. Trevor Stigers is a 3* DE prospect. Did we get a net increase in talent when Titus left? Those rankings say so. In reality, who the hell knows? There is no way to know without it playing out.
And, as others have pointed out, any time you get a starter back, you are bettering your football team. The player should be better able to make good things happen, due less to an increase in talent and more to an increase in experience.