What is changing in the industry has more to do about 'virtualisation' than it has to do about 'architecture'. Many enterprises are going for virtualisation on VMware, and yes, VMware does only run on x86. Therefor we find that Windows and Linux are on the rise... not because they're perse so good , but because they are easy to virtualise, which cuts TCO. That said, Solaris CAN run as well on VMware but currently is not (yet ?) considered probably for the simple reason that most IT managers simply don't realise that it does run on VMware.
Big iron RISC servers during an economy in trouble obviously are going to see a decline because of their cost. But when workloads need more than 4 CPU cores and the best in IO performance then Windows simply can't do what Solaris, AIX and HP-UX can do. Linux can do a lot but it also has it's limits, not in the least because Linux isn't an OS, it's a kernel. Build around it we have SuSe , Redhat, Ubuntu ... So choices need to be made because although these are very much alike, in an enterprise you want them to be IDENTICAL to keep down the cost of administration.
So Big Iron isn't going away any time soon, what is going away is 'Midrange iron' and it's being replaced by VMware on Blades, but only for the 'small' workloads, essentially Tier 1.