This 'new' ability to only run Linux is not new. If you have access to a Power 6 or Power 7 system, and look in ASMI or on the HMC (and I presume SDMC and IVM) at the entitlements section, there has been an entitlement for both AIX and IBM I for several years. Linux has been an officially endorsed OS by IBM on PowerPC for at least 7 years (they have had agreements with Slackware and SuSE), and there are official distributions of RedHat and Ubuntu from those companies.
This makes this a re-announcement of an existing policy, probably to remind some existing PowerPC shops that they can stick with Power rather than moving to another processor, even if they are switching OS. I very much doubt that the product announced will significantly differ from other systems that will still run AIX and IBM I.
This does not give any new reinforcement that policy that you bring up in every discussion about PowerPC or AIX. Both AIX and PowerPC will be here for some time still. That link lookes older each time I look at it.
Now. I'm not going to argue with the fact that AIX (along with all proprietary UNIX systems) is on the downward side of the popularity curve, and I do not think that PowerPC development is in a good place at the moment. It's expensive to build new generations of any processor, and I think that IBM is really thinking hard about what to do with the PowerPC line, at least in high end servers. Sometimes I wonder whether IBM really wants to remain in the hardware business at all (products that have been sold include their printer division, their storage division, the desktop and laptop PC business, and most recently their ATM and PoS business).
This policy may extend to their server systems as well. Power7+ is late according to previous product roadmaps, and there is strangely very little pre-announcement information about Power8. IBM has also made statements that their previously loss-leading HPC work has to become more commercial (probably one of the reasons why IBM pulled out of Blue Waters), which means that future generations of IH HPC systems are at risk.
But one of the effects of there being a creditably competitor to Intel processors is that makes Intel aggressively pursue new processors. Once they are only competing with themselves (remember, AMD need chipfabs like IBM to create their products, because they cannot fabricate processors themselves), then the rate of product development will slow significantly, as Intel would want to get more return on their investment.
I am really not looking forward to a point where the only processor game in town is X86 derived, and that is looking like a possibility within a decade unless ARM moves upward.