I don't think any of Kepa, Christensen or Rudiger are poor players, I just think none of them are good enough for a title challenging side.
I think one of Zouma or Tomori (probably Tomori if the plan is to have 2x ball playing defenders) is good enough to be CB2. Kepa is not as good as Cudicini, who we had as our GK2 for a while (and who, before his motorcycle crash, was a great GK1).
I like Alonso as a wing back. He lacks pace but he has a mighty strike on him and takes a wicked free kick. I don't think he is good enough defensively as a left back.
As many have said, I think to challenge we'll need to replace 3 out of 5 of the "back 5". Our current defence is not even a top 4 defence.
Thank god for Leicester. Chelsea v Leicester in the battle for who can bottle it harder!