I agree with what you're saying in terms of our goals conceded number, it's highly unlikely we'll manage to overperform on xga like we did last season. I'm assuming that's what's you've looked at with your predictions? What also needs factoring in is that we also underperformed on xg, so the net over performance was only actually 4 goals.
For me you can't look at the defensive stats in isolation, as an indicator of our drop off.
Analytically speaking a top of the bottom half finish looks likely. Admittedly the eye test says our attack currently looks toothless, but that was without McGoldrick, Mousset and the surely inevitable addition.
Basically, I think we're being a bit overly pessimistic about how the season is likely to go. Especially considering that we have much better squad depth, particularly defensively. We also start the season with Berge, who in my opinion is a huge huge upgrade on anything we had this time last year.