While there are plenty of good non-Brooks saddles, I don't think there is saddle that is just like the Brooks B17, but made out of synthetic materials. I think the many qualities of the Brooks B17 comes the very shape and choice of materials.

I have abandoned the search for a B17 substitute for several reasons:

For me, the Brooks B-17 is a perfect fit: I can ride mine all day without problems. I even find them comfortable from day one. I also find them long lasting.

There is actually a chance that I can buy exactly the same saddle shape if I need another saddle in 10 years or so.

I used to ride a Turbomatic saddle (can't remember the exact model) but when the synthetic leather was worn, I could no longer buy the same saddle. The new Turbomatic model had another shape and stitches, and gel pads and what not. Then I tried San Marco Rolls saddles; they where OK and cheap while not long lasting. But then they stopped making Rolls saddles (it has later reappeared as an expensive retro-saddle). Many synthetic saddles seems only to be in production for some years, before marketing forces dictate a change, since for many people an "old model = boring".

It can be a slow, expensive and painful experience to change saddle models. It is something I would rather avoid, so I stick with my Brooks B17's. Yes, I wish they where lighter etc., but to me their shortcomings are minor compared to their main advantages; they are comfortable, and the model is very likely in production in 10 or 20 years time too.