Good evening Alastair, that's a really interesting question and I hope the active community here ( although understandably Maya Biased ) can discuss this out a little.
Personally I'm not a veteran Softimage user but I've worked alongside many loyal Softimage users and on occasion supported the software on bespoke projects so hopefully I can throw in a couple of pence here.
What initially popped into my head when I heard the news was a shocking "but,but,but what about ICE!!?" and got into an acceptance that many of the greatest technologies of the last century have indeed ended up in a drawer somewhere and ICE just got registered as one of those.
Not being an in depth user I have tried it extensively and I can not say I got up to 5 reasons to leave Maya for it but these 2 hold absolute true in my mind:
--[ ICE
Simulation as never before seen, intuitive workflow, easy to use, with razzle and dazzle only a click away
--[ The node graph itself
Houdini tries, Maya emulates, Nuke utilizes, but out of all the 3D software I've tried the way Softimage implemented the node graph both visually and to detail made it one of those I could pick up and it just made sense to work in it, straightforward without complication and one could just get carried away with creating things in an almost self-explanatory fashion.
As for your other points, hopefully these alternates can fill the gap in a large user base which preferred the approach used in the newly retired software.
--[ Render pass and partition system. It is absolutely robust and does all you expect. Indespensible
When it first came out I was equally as impressed but as time passed other software caught up, the render pass system in Maya and Houdini (using v-ray,mental ray, Arnold, and Mantra for reference) all offer extended render pass control both in a simplified UI way, a more granular control approach from within the editors, and full fanatical control through the command line, variable evaluation, and environment control.
--[ Live operator stack and construction history
Maya stores extended history in a way that it ripples through the chain if you alter things retroactively, but I'm not sure you will find a similar dynamic and accurate "retro-tweaking" capabilities. You can, but will need to alter your workflow completely and there is not much to the visual side of things to that regard.
--[ Animation, modelling and rigging toolsets. They are peerless.
a) Maya, with all it's fancy ways, is primarily an animation software with a lot of extra features. Surely a lot of people use it for many other things these days and whole infrastructures have been built on the sole fact of how versatile the software is, but it is still a fact that in the same way as the iPhone is an iPod music player, padded with limitless features on top, Maya as well is for animating and if you need anything else then you can also do that using the same software ( just so happens that Autodesk are quite skilled in that other stuff so it has kind of surpassed the animation side of things ). The irony of the whole thing is of course the fact the toolset is so insanely good it hasn't received any updates since early last decade, fancy additions like Trax editors and such but the core toolset is still the power tool it is, with the exact same features as they were almost 10 years ago.
b) Among the most impressive features of Maya 2014 is a newly reworked modelling toolkit, right up there raising flags as one of the more desirable features in the new release. You won't find much praise to that regard online since when you search for modelling in Maya you will stumble upon people discussing old methods but as far as I can see it kind of just does the work without complications. 2014 for example is the only version of Maya where I've never felt the urge to install third party modelling utilities as the most desired features appear to be mostly integrated by now.
c) If you are looking for a new rigging solution, and are going to be spending time on R&D anyway, something tells me Fabric is worth taking time on. It's not meant for rigging, rigging there within is kind of like a happy second feature but all I can say it install Maya, set up the Fabric Engine, and watch in awe as the rigging industry revolts.
Hope this is along the lines of the kind of feedback you are after, in short; Softimage will be missed but alternatives exist, having to make the adjustment sucks of course but hopefully you'll be part of the Softimage user base which brings the good experiences that software gave us to a new user base which is always aching for something different.