Okay, after further looking into this, I'm pretty much set on cancelling my Pro membership. I feel Ipernity's business model is much more appealing (a community that finances its own services - what's not to like?). I'm fully aware of the fact that there's no telling how Ipernity would scale, but they have very sound expertise behind the platform as well as an active core team, so I don't see why it shouldn't work out in the long run (except of course for financial reasons).
A key aspect in all this is that I really don't need most of Flickr's Pro "benefits" - ad-free content presentation and unlimited space/traffic are nice, but other aspects (community interaction being one major thing) really aren't up to today's standard or not useful to me, and all the merchandising stuff is more of a nuisance than an asset in my eyes. Ipernity is much more tightly nit as a community, and the feature set appeals a lot more to me than Flickr's. Crucially, except for "unlimited" storage (100GB are more than enough for me), you actually get more for your money when using Ipernity's Standard plan - including a personalised page layout that may even enable me to replace/drop my by now totally defunct personal website ... All in all, I think it's well worth the risk going for the test Club membership (one year at a hugely reduced rate with full benefits of the Standard plan) for me.
And there's another thing: I usually only present my images on Cameraderie and maybe mu-43.com - I don't need or want a bigger audience that's just after the trendy stuff I'm not interested in shooting.
I know that in the mid-term, this switch will most probably mean leaving Flickr altogether (I don't have the time and stamina to maintain more than one collections - not that I have done a good job at maintaining Flickr, to be honest). We'll see how it goes and how well I can live with all the restrictions and downsides of a free account. But the moment it gets bad, I'll pull everything off Flickr and be gone.
One week left to ponder ...
M.