I purposefully went for a 657 rather than an 857, when I went looking, and bought a 657 in 2016. I wrote the below at the time

And so on to the 657. It gave way to the 857 in the model line-up. At a glance you would struggle to tell them apart.

However, during the transformation from 6 to 8 it gained some extra paunch in the form of 2mm larger in diameter, plus a tegimented case, a re-orientation of the date window, copper sulphate capsule and a matte dial. The tegiment is a good idea, the others are meh, for me.
The 41mm diameter 657 is plenty big enough. I personally cannot see that moving from 41mm diameter to 43mm diameter is progress in anyone's book. Unless you are rather too pumped by the gym or the donut stand.

And why take away that glorious deep gloss dial? There is nothing blacker than a lovely deep shiny piano black. The flatness of matte renders the dial grey by comparison (see above matt EZM1 for comparison). I have spent a few TZ GTG evenings looking very admiringly at Mike's 556i dial (before he moved it on), and that was the primary inspiration for me to seek out this version. That black is much nicer. And a gloss dial is not even an option on the 85* series.

I suspect the main reason for the up-spec is to differentiate in price for the new model.
Compare the 556A to the equivalent 856, almost equivalent watches except the size and tech referred to above.

Page and Cooper have inc-VAT prices of £695 for the 556A on leather. The 856 on leather? £1145. That is a £450 lift or 65% more than the smaller model.

For what??? Tegimenting???Technically the only other uplift between the two is the addition of CuSO4 capsule. No addition to WR.
It seems clear that Sinn wanted to keep a basic cheapie, but remove the option of a cheaper model with a bezel.

So rendering the 657 obsolete was without doubt a means for Sinn to add to the arms race of technology and increase the price of the replacement model a looooooong way.

I would have thought there is still room for this model in the line-up. What is wrong with a 657 or a 557 (not that this ever existed, but I mean a 556 equipped with a bezel)? I would have thought it might be popular, but then again, it may damage 857 sales. Perhaps that is why we do not have it.

It seemed very clear to me that the 85* range was made in order to upsell. Add a few techy features, and fluff the price by a long way.
Sinn have some very useful tech. Tegiment is definitely one.
But The Copper Sulphate? Ar filling? Diapal? These are techs that are solutions to non-existant problems, imho. They are also ways to ensure that you have to go to Sinn for your servicing........