Well, the way I'm looking at it is simply that there's a reason we tried for the likes of Schneiderlin, McCarthy and co., and there's a reason we turned Dier into a DM to begin with: if Poch was satisfied with Bentaleb's performances at DM (alongside Mason last season), then a radical move like that wouldn't have been necessary. As we're now seeing, it appears that Poch can seriously improve the attributes of any player: perhaps Bentaleb's defensive game wasn't something Poch thought could be improved quickly enough to persist with him as a DM heading into this season, or perhaps there's another reason for the DM shift we're not entirely seeing.
Bentaleb has skills,no doubt: skills that will make him beastly in the future if further improved upon. He passes well (long and short), he tackles and hustles reasonably well, he dribbles well and he's aggressive and fearless in the challenge. However, from my position earlier in the summer (when I think I actually advocated his usage as a DM), I now feel like he'd be better used as a B2B midfielder like Alli: perhaps rotating with him when the lad (inevitably) tires heading into the second half of the season, or playing in his position with Alli shifted further up into the AM position. Either way, I don't see him as a 'sitting' DM a la Dier: and since we don't have a like-for-like replacement for the lad (or any replacement, really, unless we want to constrain Dembele again by making him play there), I feel that's a smaller hole that needs filling. Not as urgently as CF backup for Kane, but still a hole nonetheless. Hence Vogt (who's still only 24, has played alongside Wimmer, and has, iirc, a good long pass on him to boot).