I don't see how that is "a more defensive minded player behind our pivot"? Are you describing Mason and Eriksen as our current "pivot"?
I think we're playing a style that is fairly demanding, physically, mentally and not least tactically. Like you say a lot of our width comes from our full backs, with defensive cover for their attacking movement coming from one of the two deeper midfielders (Bentaleb and Mason this season at least). Particularly when both full backs attack at the same time. That's quite different from a (imo) simpler model where the full backs are essentially told not to venture too far forward at the same time and all the full back has to do to balance things out is to look at the other full back and make a decision.
That's quite a few difficult decisions to be made on when to attack and when to defend. Mason seems to have the most attacking freedom with Bentaleb more defensive responsibility when we attack. So it's been up to Rose, Bentaleb and Walker/Dier/Chiriches/Naughton to balance the attacking movement with defensive responsibility to not leave the two centre backs isolated.
This is combined with a (at times) high pressing system where we try to win the ball back quickly quite often and press high up the pitch when the opponents are building from the back at least some of the time. We're doing this with a very young front 4, and with some players not ideally suited to a pressing game like that (I'm thinking primarily of Townsend and Chadli, but whatever). Again this is more complex than a "when we lose it just run back and get behind the ball" system. Again some difficult decisions have to be made, perhaps particularly for those 2 full backs and 2 deep midfielders.
I keep coming back to two points:
1. We're trying to do this with a very young team. Both the full backs and deep midfielders are young, inexperienced, and sometimes playing in relatively new positions (Dier, Chiriches, Mason - even Bentaleb). The attacking 4 is also young and learning when to press, when to drop back etc - more important when we press against teams building from the back than when counter attacked on, but still important. Even without any new signings (though we will make new signings) I would expect us to defend better next season - a year more experienced and a year of learning Pochettino's system. There will also be some continuity, for the first time in a handful of years.
2. We have a very talented player in Bentaleb with few (if any) real weaknesses in his game playing the deepest midfield role. It might be that we can find a short term upgrade on him, but I really doubt we can find someone that's currently better that also has his obvious talent and (imo) extreme potential. Those players quite simply don't sign for Spurs.
This is as situation where I think it's for the best to accept some short term pain for a long term gain. I don't think we should look to replace a player as talented as him in the starting line-up. Instead I think we should persevere as he continues to develop and learn to become the player he seems to be likely to develop into. This is key for our progress, developing talented players into big stars. And that takes a bit of short term pain to make happen.