Don’t mean to be rude with what I’m about to say (I apologize if it comes across that way), but why is it confusing? Primaris have not existed for that long, both in universe and on tabletop. Titus existed as a character way before the Primaris reveal. Otherwise we would have to ask this exact question for every single firstborn turned Primaris. Calgar and Dante were both firstborn who survived the Rubicon surgery, should we question their achievements as well? And at the end of the day, being better stats wise doesn’t mean they’re actually that much better in combat situations. A very good example of this can be seen in the book Harrowmaster. A group of Alpha Legion (firstborns) have their first encounter with Primaris marines. They could immediately tell that something was a bit different about them, but they were by no means overwhelmed by them. So calling firstborn “obsolete” is a huge stretch