I hardly think Gundogan and Bernardo are the best examples. Both of which are trusted by Pep and play a ton of games. Both are also capable of playing in several positions so are likely to find game time throughout the season.
If you are a CB then it becomes 10x harder to get in the team because you are in a specialized position and it will take an injury crisis to get in the team and stay there.
Rodri and Grealish did join the squad knowing they had competition but Phillips realistically has one person to compete with in Rodri and Grealish whilst he is fighting for game time with Foden, Mahrez and Bernardo for a position, is playing on the wing and there is more to go around there.
I agree we aren't going to sign someone who doesn't back themselves for obvious reasons but I bet you will struggle to find a CB in world football who would back themselves to beat out Stones, Dias, Laporte and Ake.
Ask a RW for example if they fancy battling Mahrez and the answer is completely different.
Anyway, its all a moot argument. We aren't signing anyone else unless someone leaves and CB will be bottom of the pile for a while I reckon.