Well, the scriptures plainly talk about overcomers in Revelation 2 and 3. Several times, "to him who overcomes". That means there are those who may "not overcome". So it's really the scriptures that describe two categories. The same thought is present in many other portions without the specific use of the word "overcomer".
But to your point: YES! there is the overcoming of the world, our faith, our believing in the Son of God! Yes, that is a victory, but it is just the initial one, and not the only one. The use of the word "
overcomes" here does not negate the use of "
overcome" in other contexts, and concerning other requirements. It's one thing to overcome unbelief and the world and believe in the Son of God, but another to overcome lukewarmness, Jezebel, Nicolaitans, lost of first love, deadness, etc, etc.
Yes, biology and family teaches us this also. We were all sperm one day, and we all
overcame the race against all other sperm and reached the egg successfully, .....
, we overcame the world of sperm and got their first, we believed and we were born!
But now we have to grow, and mature, and run the human race, and keep all of dadīs requirements to the end, and
overcome all the other obstacles, then we might receive the reward of dadīs inheritance.
The first overcome does not negate the second.