Krystian Bielik has been impressing for the reserves/youth at CB so there's younger players that I'm sure Wenger would like to promote eventually. It's a case of timing here and I don't think you can totally fault Wenger on team selection sometimes (he has a knack for shoe-horning in the wrong guys but it's different here).
CB is such a finicky position to handle, especially when it comes to someone like Wenger who doesn't focus much on our defensive aspects, but we found our best CB pairing in Mertesacker and Koscielny after years of trying numerous other pairings. To break that up for Calum would have been just as ludicrous as our transfer dealings this summer.
Now that position has been blown wide open with Per/Gab out and Kosh being 30+ now. Chambers has to take his chance, and well enough to establish himself.