I'm not so sure, yes there will be some buyers for the Platinum version, more for a gold version but surely these sales will be minimal year on year compared to the interest in the SS version.
However, the question has to be asked, will there be 000's of orders for a 321 in SS on an annual basis, possibly in the first year or two but beyond that I'm not convinced. I understand that for a lot of people having a Speedmaster with a 321 movement is going to be a grail watch but its not a limited edition, you will be able to order one for many years to come.
The other thing to consider will be the specification of the stainless steel version, a traditional Speedy dial protected by a sapphire crystal, a SS case and bracelet, aluminium or ceramic bezel (my guess ceramic) and a sapphire crystal display back to show off the gorgeous movement. A special presentation box and some additions maybe a NATO strap. I don't believe that it will contain any precious metals so why would the cost be beyond that of the 50th.
I hear many occasions that the SS 321 will cost around 20k but why? Without doubt the SS 321 will be the premium Moonwatch in Omega's line up, but why should it cost in excess of 50% more than the current top of the range SS Moonwatch, the only addition is likely to be the 321 movement and a ceramic bezel (possibly), if they add or change too much Omega risk losing the watch's identity, and that is something that I think we would all want to retain.